Gallery: Fix property names referred in the Ribbon class.

ui/file_manager/gallery/js/ribbon.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.
 
 'use strict';
 
 /**
  * Scrollable thumbnail ribbon at the bottom of the Gallery in the Slide mode.
  *
  * @param {Document} document Document.
@@ skipping 326 matching lines @@
 };
 
 /**
  * Content change handler.
  *
  * @param {Event} event Event.
  * @private
  */
 Ribbon.prototype.onContentChange_ = function(event) {
   var url = event.item.getEntry().toURL();
-  this.remapCache_(event.oldUrl, url);
+  this.remapCache_(event.oldEntry.toURL(), url);
 
   var thumbnail = this.renderCache_[url];
   if (thumbnail && event.metadata)
     this.setThumbnailImage_(thumbnail, event.item.getEntry(), event.metadata);
 };
 
 /**
  * Update the thumbnail element cache.
  *
  * @param {string} oldUrl Old url.
  * @param {string} newUrl New url.
  * @private
  */
 Ribbon.prototype.remapCache_ = function(oldUrl, newUrl) {
   if (oldUrl != newUrl && (oldUrl in this.renderCache_)) {
     this.renderCache_[newUrl] = this.renderCache_[oldUrl];
     delete this.renderCache_[oldUrl];
   }
 };