Proof of concept using chrome.streamsPrivate API

This method captures all application/pdf streams, loads the viewer
and passes the stream to the PDF.js viewer.

This commit shows a proof of concept using the chrome.streamsPrivate API.

Advantages of new method:
- Access to the response body of the original request, thus fewer
  network requests.
- PDFs from non-GET requests (e.g. POST) are now supported.
- FTP files are also supported.

Possible improvements:
- Use declared content scripts instead of dynamic chrome.tabs.executeScript.
  This allows the extension to render the viewer in frames when the
  extension is disallowed to run executeScript for the top URL.
- Use chrome.declarativeWebRequest instead of webRequest, and replace
  background page with event page (don't forget to profile the
  difference & will the background/event page still work as intended?).

web/chromecom.js

@@ -0,0 +1,46 @@
+/* -*- Mode: Java; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* Copyright 2013 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 */
+'use strict';
+
+var ChromeCom = (function ChromeComClosure() {
+  return {
+    /**
+     * Creates an event that the extension is listening for and will
+     * asynchronously respond by calling the callback.
+     * @param {String} action The action to trigger.
+     * @param {String} data Optional data to send.
+     * @param {Function} callback Optional response callback that will be called
+     * with one data argument. When the request cannot be handled, the callback
+     * is immediately invoked with no arguments.
+     */
+    request: function(action, data, callback) {
+      var message = {
+        action: action,
+        data: data
+      };
+      if (!chrome.runtime) {
+        console.error('chrome.runtime is undefined.');
+        if (callback) callback();
+      } else if (callback) {
+        chrome.runtime.sendMessage(message, callback);
+      } else {
+        chrome.runtime.sendMessage(message);
+      }
+    }
+  };
+})();