[Gallery app] Make the background page use BackgroundBase class

ui/file_manager/externs/gallery_background.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.
 
 /**
  * This is a type to let closure compiler recognize extended methods to Window
  * instance at gallery/js/gallery.js.
  * @constructor
  * @extends {Window}
  */
 function GalleryWindow() {}
 
 /**
- * @param {!BackgroundComponents} backgroundComponents Background components.
+ * @type {Promise}
  */
-window.initialize = function(backgroundComponents) {};
-
-/**
- * @param {!Array.<!Entry>} entries Array of entries.
- */
-window.loadEntries = function(entries) {};
+window.initializePromise;
 
 /**
  * This definition is required by
  * ui/file_manager/file_manager/common/js/util.js.
  * @type {string}
  */
 Window.prototype.appID;