[Galley] Fix an exception in clicking a thumbnail

chrome/browser/resources/file_manager/foreground/js/photo/ribbon.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';
 
 /**
  * Scrollable thumbnail ribbon at the bottom of the Gallery in the Slide mode.
  *
  * @param {Document} document Document.
@@ skipping 287 matching lines @@
     var img = cached.querySelector('img');
     if (img)
       img.classList.add('cached');
     return cached;
   }
 
   var thumbnail = this.ownerDocument.createElement('div');
   thumbnail.className = 'ribbon-image';
   thumbnail.addEventListener('click', function() {
     var index = this.dataModel_.indexOf(item);
+    this.selectionModel_.beginChange();

[hirono, 2014/02/13 04:57:07]

I'm not sure why the pair of methods fix the exception. Could you explain?

     this.selectionModel_.unselectAll();
     this.selectionModel_.setIndexSelected(index, true);
+    this.selectionModel_.endChange();
   }.bind(this));
 
   util.createChild(thumbnail, 'image-wrapper');
 
   this.metadataCache_.get(item.getEntry(), Gallery.METADATA_TYPE,
       this.setThumbnailImage_.bind(this, thumbnail, item.getEntry()));
 
   // TODO: Implement LRU eviction.
   // Never evict the thumbnails that are currently in the DOM because we rely
   // on this cache to find them by URL.
@@ skipping 37 matching lines @@
  * @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];
   }
 };