[Wallpaper Picker] Cleaned UI artefacts in Surprise Me Mode.

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 27 matching lines @@
       imageEl.classList.add('thumbnail');
       cr.defineProperty(imageEl, 'offline', cr.PropertyKind.BOOL_ATTR);
       imageEl.offline = this.dataItem.availableOffline;
       this.appendChild(imageEl);
       var self = this;
 
       switch (this.dataItem.source) {
         case Constants.WallpaperSourceEnum.AddNew:
           this.id = 'add-new';
           this.addEventListener('click', function(e) {
-            $('wallpaper-selection-container').hidden = false;
+            var checkbox = $('surprise-me').querySelector('#checkbox');
+            if (!checkbox.classList.contains('checked'))
+              $('wallpaper-selection-container').hidden = false;
           });
           break;
         case Constants.WallpaperSourceEnum.Custom:
           var errorHandler = function(e) {
             console.error('Can not access file system.');
           };
           var wallpaperDirectories = WallpaperDirectories.getInstance();
           var getThumbnail = function(fileName) {
             var setURL = function(fileEntry) {
               imageEl.src = fileEntry.toURL();
@@ skipping 220 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
   };
 });