Disable surprise me automatically if the current wallpaper was set by a third party application.

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 185 matching lines @@
    */
   WallpaperManager.prototype.toggleSurpriseMe_ = function() {
     var self = this;
     var checkbox = $('surprise-me').querySelector('#checkbox');
     var shouldEnable = !checkbox.classList.contains('checked');
     WallpaperUtil.saveToStorage(Constants.AccessSurpriseMeEnabledKey,
                                 shouldEnable, true, function() {
       if (chrome.runtime.lastError == null) {
           if (shouldEnable) {
             checkbox.classList.add('checked');
+            // Hides the wallpaper set by message if there is any.
+            $('wallpaper-set-by-message').textContent = '';
           } else {
             // Unchecking the "Surprise me" checkbox falls back to the previous
             // wallpaper before "Surprise me" was turned on.
-            self.setSelectedWallpaper_(self.wallpaperGrid_.activeItem_);
-            self.onWallpaperChanged_(self.wallpaperGrid_.activeItem_,
-                                     self.currentWallpaper_);
+            if (self.wallpaperGrid_.activeItem) {
+              self.setSelectedWallpaper_(self.wallpaperGrid_.activeItem);
+              self.onWallpaperChanged_(self.wallpaperGrid_.activeItem,
+                                       self.currentWallpaper_);
+            }
             checkbox.classList.remove('checked');
           }
           $('categories-list').disabled = shouldEnable;
           $('wallpaper-grid').disabled = shouldEnable;
         } else {
           // TODO(bshe): show error message to user.
           console.error('Failed to save surprise me option to chrome storage.');
         }
     });
   };
@@ skipping 871 matching lines @@
       }
       this.wallpaperGrid_.dataModel = wallpapersDataModel;
       if (selectedItem) {
         this.wallpaperGrid_.selectedItem = selectedItem;
         this.wallpaperGrid_.activeItem = selectedItem;
       }
     }
   };
 
 })();