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

chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_images_grid.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.
 
 cr.define('wallpapers', function() {
   /** @const */ var ArrayDataModel = cr.ui.ArrayDataModel;
   /** @const */ var Grid = cr.ui.Grid;
   /** @const */ var GridItem = cr.ui.GridItem;
   /** @const */ var GridSelectionController = cr.ui.GridSelectionController;
   /** @const */ var ListSingleSelectionModel = cr.ui.ListSingleSelectionModel;
@@ skipping 205 matching lines @@
      *     wallpaperInfo The information of the wallpaper to be set active.
      */
     activeItem_: undefined,
     set activeItem(activeItem) {
       if (this.activeItem_ != activeItem) {
         this.activeItem_ = activeItem;
         this.updateActiveThumb_();
       }
     },
 
+    get activeItem() {
+      return this.activeItem_;
+    },
+
     /**
      * A unique ID that assigned to each set dataModel operation. Note that this
      * id wont increase if the new dataModel is null or empty.
      */
     dataModelId_: 0,
 
     /**
      * The number of items that need to be generated after a new dataModel is
      * set.
      */
@@ skipping 150 matching lines @@
       // The active thumbnail maybe deleted in the above redraw(). Sets it again
       // to make sure checkmark shows correctly.
       this.updateActiveThumb_();
     }
   };
 
   return {
     WallpaperThumbnailsGrid: WallpaperThumbnailsGrid
   };
 });