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

remoting/webapp/base/js/message_window_helper.js

Index: remoting/webapp/base/js/message_window_helper.js
diff --git a/remoting/webapp/base/js/message_window_helper.js b/remoting/webapp/base/js/message_window_helper.js
index d608a034829f54f54688f95b9729821308438218..51d63c0d41ba7f666e6480736953a81e759d215f 100644
--- a/remoting/webapp/base/js/message_window_helper.js
+++ b/remoting/webapp/base/js/message_window_helper.js
@@ -121,7 +121,9 @@ remoting.MessageWindow = function(options) {
   };
 
   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),
@@ -130,6 +132,15 @@ remoting.MessageWindow = function(options) {
 };
 
 /**
+ * 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).
  */