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.

gulpfile.js

@@ -79,6 +79,13 @@ function checkChromePreferencesFile(chromePrefsPath, webPrefsPath) {
   var webPrefs = JSON.parse(fs.readFileSync(webPrefsPath).toString());
   var webPrefsKeys = Object.keys(webPrefs);
   webPrefsKeys.sort();
+  var telemetryIndex = chromePrefsKeys.indexOf('disableTelemetry');
+  if (telemetryIndex >= 0) {
+    chromePrefsKeys.splice(telemetryIndex, 1);
+  } else {
+    console.log('Warning: disableTelemetry key not found in chrome prefs!');
+    return false;
+  }
   if (webPrefsKeys.length !== chromePrefsKeys.length) {
     return false;
   }