Make the third party wallpaper syncable through different devices.

chrome/browser/resources/chromeos/wallpaper_manager/js/constants.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.
 
 
 /** @const */ var Constants = {
   /**
    * Key to access wallpaper rss in chrome.storage.local.
    */
   AccessLocalRssKey: 'wallpaper-picker-surprise-rss-key',
@@ skipping 73 matching lines @@
    * Suffix to append to file name if it is a thumbnail.
    */
   CustomWallpaperThumbnailSuffix: '_thumbnail',
 
   /**
    * Wallpaper directory enum.
    */
   WallpaperDirNameEnum: {
       ORIGINAL: 'original',
       THUMBNAIL: 'thumbnail'
-  }
+  },
+
+  /**
+   * The filename prefix for a third party wallpaper.
+   */
+  ThirdPartyWallpaperPrefix: 'third_party_'
 };