Add externs for gallery.

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.
  */
 window.initialize = function(backgroundComponents) {};
 
 /**
  * @param {!Array.<!Entry>} entries Array of entries.
  * @param {!Array.<!Entry>} selectedEntries Array of selected entries.
  */
 window.loadEntries = function(entries, selectedEntries) {};
+
+/**
+ * This definition is required by
+ * ui/file_manager/file_manager/common/js/util.js.
+ * @type {string}
+ */
+Window.prototype.appID;