Add script files of the separated Gallery.app.

chrome/browser/resources/gallery/js/gallery_item.js

 // Copyright (c) 2012 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';
 
 /**
  * Object representing an image item (a photo or a video).
  *
  * @param {FileEntry} entry Image entry.
@@ skipping 208 matching lines @@
         newFileName,
         {create: false, exclusive: false},
         onExists,
         function() {
           this.entry_.moveTo(parentDir, newFileName, onRenamed, onError);
         }.bind(this));
   }.bind(this);
 
   this.entry_.getParent(moveIfDoesNotExist, onError);
 };