Add externs for gallery.

ui/file_manager/externs/gallery_foreground.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.
 
 /**
  * BackgroundComponents.
  * @constructor
  * @struct
  */
 function BackgroundComponents() {
   /**
    * @type {!Object.<string, string>}
    */
   this.stringData;
 
   /**
    * @type {!VolumeManager}
    */
   this.volumeManager;
 }
+
+/**
+ * File Manager. This definition is required by
+ * ui/file_manager/file_manager/foreground/js/ui/file_manager_dialog_base.js.
+ *
+ * @constructor
+ * @struct
+ */
+function FileManager() {}
+
+/**
+ * This definition is required by
+ * ui/file_manager/file_manager/common/js/util.js.
+ * @type {string}
+ */
+Window.prototype.appID;
+
+/**
+ * @type {string}
+ */
+Entry.prototype.cachedUrl;