Fix unexpected error message when quickly select different wallpapers

chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_manager.js

 // Copyright (c) 2013 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.
 
 /**
  * WallpaperManager constructor.
  *
  * WallpaperManager objects encapsulate the functionality of the wallpaper
  * manager extension.
  *
@@ skipping 489 matching lines @@
 
           // Falls back to request wallpaper from server.
           if (self.wallpaperRequest_)
             self.wallpaperRequest_.abort();
 
           self.wallpaperRequest_ = new XMLHttpRequest();
           self.progressManager_.reset(self.wallpaperRequest_, selectedGridItem);
 
           var onSuccess = function(xhr) {
             var image = xhr.response;
-            chrome.wallpaperPrivate.setWallpaper(image, selectedItem.layout,
-                wallpaperURL,
-                self.onFinished_.bind(self, selectedGridItem, selectedItem));
-            self.currentWallpaper_ = wallpaperURL;
-            WallpaperUtil.saveWallpaperInfo(wallpaperURL, selectedItem.layout,
-                                            selectedItem.source);
-            self.wallpaperRequest_ = null;
+            if (image) {

[flackr, 2013/09/06 15:44:00]

onSuccess can actually be called if there's no response? Can you instead handle
this in fetchURL and avoid calling onSuccess if there's no response? Also,
shouldn't no response trigger the error handler?

+              chrome.wallpaperPrivate.setWallpaper(image, selectedItem.layout,
+                  wallpaperURL,
+                  self.onFinished_.bind(self, selectedGridItem, selectedItem));
+              self.currentWallpaper_ = wallpaperURL;
+              WallpaperUtil.saveWallpaperInfo(wallpaperURL, selectedItem.layout,
+                                              selectedItem.source);
+              self.wallpaperRequest_ = null;
+            }
           };
           var onFailure = function() {
             self.progressManager_.hideProgressBar(selectedGridItem);
             self.showError_(str('downloadFailed'));
             self.wallpaperRequest_ = null;
           };
           WallpaperUtil.fetchURL(wallpaperURL, 'arraybuffer', onSuccess,
                                  onFailure, self.wallpaperRequest_);
         });
         break;
@@ skipping 458 matching lines @@
           }
         }
       }
       this.wallpaperGrid_.dataModel = wallpapersDataModel;
       this.wallpaperGrid_.selectedItem = selectedItem;
       this.wallpaperGrid_.activeItem = selectedItem;
     }
   };
 
 })();