[AppRemoting] Break out AppRemoting shared module (re-land).

remoting/webapp/base/js/message_window_helper.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 /** @suppress {duplicate} */
 var remoting = remoting || {};
 
 /** @constructor */
@@ skipping 103 matching lines @@
   /** @param {chrome.app.window.AppWindow} appWindow */
   var onCreate = function(appWindow) {
     that.setWindow_(/** @type {Window} */(appWindow.contentWindow));
     var onLoad = function() {
       appWindow.contentWindow.postMessage(message_struct, '*');
     };
     appWindow.contentWindow.addEventListener('load', onLoad, false);
   };
 
   var htmlFile = options.htmlFile || 'message_window.html';
-  chrome.app.window.create(htmlFile, windowAttributes, onCreate);
+  chrome.app.window.create(
+      remoting.MessageWindow.htmlFilePrefix + htmlFile,
+      windowAttributes, onCreate);
 
   if (duration != 0) {
     this.timer_ = window.setTimeout(this.onTimeoutHandler_.bind(this),
                                     duration);
   }
 };
 
 /**
+ * This string is prepended to the htmlFile when message windows are created.
+ * Normally, this should be left empty, but the shared module needs to specify
+ * this so that the shared HTML files can be found when running in the
+ * context of the app stub.
+ * @type {string}
+ */
+remoting.MessageWindow.htmlFilePrefix = "";
+
+/**
  * Called when the timer runs out. This in turn calls the window's
  * timeout handler (if any).
  */
 remoting.MessageWindow.prototype.onTimeoutHandler_ = function() {
   this.close();
   if (this.onTimeout_) {
     this.onTimeout_();
   }
 };
 
@@ skipping 129 matching lines @@
 
 /**
  * Cancel the current connection and close all app windows.
  *
  * @param {number} result The dialog result.
  */
 remoting.MessageWindow.quitApp = function(result) {
   remoting.MessageWindowManager.closeAllMessageWindows();
   window.close();
 };