Add opt-out telemetry to the Chrome extension

Privacy policy: https://github.com/Rob--W/pdfjs-telemetry#privacy-policy

Unit tests (offline):

```
node test/chromium/test-telemetry.js
```

Server tests (requires that Nginx is installed):

```
git clone https://github.com/Rob--W/pdfjs-telemetry.git
cd pdfjs-telemetry/
python testserver.py TestHttp TestHttps
```

Integration test (extension + server):

- Build the extension
- Edit build/chromium/telemetry.js and remove the check for
  chrome.runtime.id.
- Start Chrome (preferably a new profile):
  chromium --user-data-dir=/tmp/pdftest --no-first-run
- Open chrome://net-internals#events
- Visit chrome://extensions and enable Developer mode.
- Load unpacked extension, select build/chromium.
- Go to the chrome://net-internals tab and filter on pdfjs.robwu.nl.
- Click on URL_REQUEST and verify that the server replied with 204.
- Reload the extension.
- Verify that chrome://net-internals did not contain a new log request.

extensions/chromium/pdfHandler.html

@@ -22,3 +22,4 @@ limitations under the License.
 <script src="pdfHandler-vcros.js"></script>
 <script src="pageAction/background.js"></script>
 <script src="suppress-update.js"></script>
+<script src="telemetry.js"></script>

extensions/chromium/preferences_schema.json

@@ -88,6 +88,12 @@
         4
       ],
       "default": 0
+    },
+    "disableTelemetry": {
+      "title": "Disable telemetry",
+      "type": "boolean",
+      "description": "Whether to prevent the extension from reporting the extension and browser version to the extension developers.",
+      "default": false
     }
   }
 }

extensions/chromium/telemetry.js

@@ -0,0 +1,162 @@
+/*
+Copyright 2016 Mozilla Foundation
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+/* globals chrome, crypto, Headers, Request */
+
+(function() {
+  'use strict';
+  // This module sends the browser and extension version to a server, to
+  // determine whether it is safe to drop support for old Chrome versions in
+  // future extension updates.
+  //
+  // The source code for the server is available at:
+  // https://github.com/Rob--W/pdfjs-telemetry
+  var LOG_URL = 'https://pdfjs.robwu.nl/logpdfjs';
+
+  // The minimum time to wait before sending a ping, so that we don't send too
+  // many requests even if the user restarts their browser very often.
+  // We want one ping a day, so a minimum delay of 12 hours should be OK.
+  var MINIMUM_TIME_BETWEEN_PING = 12 * 36E5;
+
+  if (chrome.extension.inIncognitoContext) {
+    // The extension uses incognito split mode, so there are two background
+    // pages. Only send telemetry when not in incognito mode.
+    return;
+  }
+
+  if (chrome.runtime.id !== 'oemmndcbldboiebfnladdacbdfmadadm') {
+    // Only send telemetry for the official PDF.js extension.
+    console.warn('Disabled telemetry because this is not an official build.');
+    return;
+  }
+
+  maybeSendPing();
+  setInterval(maybeSendPing, 36E5);
+
+  function maybeSendPing() {
+    getLoggingPref(function(didOptOut) {
+      if (didOptOut) {
+        // Respect the user's decision to not send statistics.
+        return;
+      }
+      if (!navigator.onLine) {
+        // No network available; Wait until the next scheduled ping opportunity.
+        // Even if onLine is true, the server may still be unreachable. But
+        // because it is impossible to tell whether a request failed due to the
+        // inability to connect, or a deliberate connection termination by the
+        // server, we don't validate the response and assume that the request
+        // succeeded. This ensures that the server cannot ask the client to
+        // send more pings.
+        return;
+      }
+      var lastTime = parseInt(localStorage.telemetryLastTime) || 0;
+      var wasUpdated = didUpdateSinceLastCheck();
+      if (!wasUpdated && Date.now() - lastTime < MINIMUM_TIME_BETWEEN_PING) {
+        return;
+      }
+      localStorage.telemetryLastTime = Date.now();
+
+      var deduplication_id = getDeduplicationId(wasUpdated);
+      var extension_version = chrome.runtime.getManifest().version;
+      if (window.Request && 'mode' in Request.prototype) {
+        // fetch is supported in extensions since Chrome 42 (though the above
+        // feature-detection method detects selects Chrome 43+).
+        // Unlike XMLHttpRequest, fetch omits credentials such as cookies in the
+        // requests, which guarantees that the server cannot track the client
+        // via HTTP cookies.
+        fetch(LOG_URL, {
+          method: 'POST',
+          headers: new Headers({
+            'Deduplication-Id': deduplication_id,
+            'Extension-Version': extension_version,
+          }),
+          // Set mode=cors so that the above custom headers are included in the
+          // request.
+          mode: 'cors',
+        });
+        return;
+      }
+      var x = new XMLHttpRequest();
+      x.open('POST', LOG_URL);
+      x.setRequestHeader('Deduplication-Id', deduplication_id);
+      x.setRequestHeader('Extension-Version', extension_version);
+      x.send();
+    });
+  }
+
+  /**
+   * Generate a 40-bit hexadecimal string (=10 letters, 1.1E12 possibilities).
+   * This is used by the server to discard duplicate entries of the same browser
+   * version when the log data is aggregated.
+   */
+  function getDeduplicationId(wasUpdated) {
+    var id = localStorage.telemetryDeduplicationId;
+    // The ID is only used to deduplicate reports for the same browser version,
+    // so it is OK to change the ID if the browser is updated. By changing the
+    // ID, the server cannot track users for a long period even if it wants to.
+    if (!id || !/^[0-9a-f]{10}$/.test(id) || wasUpdated) {
+      id = '';
+      var buf = new Uint8Array(5);
+      crypto.getRandomValues(buf);
+      for (var i = 0; i < buf.length; ++i) {
+        var c = buf[i];
+        id += (c >>> 4).toString(16) + (c & 0xF).toString(16);
+      }
+      localStorage.telemetryDeduplicationId = id;
+    }
+    return id;
+  }
+
+  /**
+   * Returns whether the browser has received a major update since the last call
+   * to this function.
+   */
+  function didUpdateSinceLastCheck() {
+    var chromeVersion = /Chrome\/(\d+)\./.exec(navigator.userAgent);
+    chromeVersion = chromeVersion && chromeVersion[1];
+    if (!chromeVersion || localStorage.telemetryLastVersion === chromeVersion) {
+      return false;
+    }
+    localStorage.telemetryLastVersion = chromeVersion;
+    return true;
+  }
+
+  /**
+   * Get the value of the telemetry preference. The callback is invoked with a
+   * boolean if a preference is found, and with the undefined value otherwise.
+   */
+  function getLoggingPref(callback) {
+    // Try to look up the preference in the storage, in the following order:
+    var areas = ['sync', 'local', 'managed'];
+
+    next();
+    function next(result) {
+      var storageAreaName = areas.shift();
+      if (typeof result === 'boolean' || !storageAreaName) {
+        callback(result);
+        return;
+      }
+
+      if (!chrome.storage[storageAreaName]) {
+        next();
+        return;
+      }
+
+      chrome.storage[storageAreaName].get('disableTelemetry', function(items) {
+        next(items && items.disableTelemetry);
+      });
+    }
+  }
+})();