Gallery.app: Fix thumbnail update of the ribbon in the slide mode.

Previously Gallery.app tried to create a new element when an image is updated,
but the new element does not added to DOM.

This CL change the behavior so that the app reuses an existing element.

BUG=385535
TEST=manually

Review URL: https://codereview.chromium.org/343873002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278405 0039d316-1c4b-4281-b951-d872f2087c98

ui/file_manager/gallery/js/ribbon.js

@@ -344,13 +344,8 @@ Ribbon.prototype.setThumbnailImage_ = function(thumbnail, entry, metadata) {
  */
 Ribbon.prototype.onContentChange_ = function(event) {
   var url = event.item.getEntry().toURL();
-  if (event.oldEntry.toURL() !== url) {
+  if (event.oldEntry.toURL() !== url)
     this.remapCache_(event.oldEntry.toURL(), url);
-  } else {
-    delete this.renderCache_[url];
-    var index = this.dataModel_.indexOf(event.item);
-    this.renderThumbnail_(index);
-  }
 
   var thumbnail = this.renderCache_[url];
   if (thumbnail && event.metadata)