Fix the issue that wallpaper changes twice when both "onChanged" and "onAlarm" are fired.

chrome/test/data/chromeos/wallpaper_manager/unit_tests/api_mock.js

 // Copyright 2014 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.
 
 
 var TestConstants = {
   wallpaperURL: 'https://test.com/test.jpg',
   // A dummy string which is used to mock an image.
   IMAGE: '*#*@#&',
   // A dummy array which is used to mock the file content.
@@ skipping 197 matching lines @@
               'source': Constants.WallpaperSourceEnum.Custom
             };
         }
         callback(items);
       },
       set: function(items, callback) {
       }
     },
     sync: {
       get: function(key, callback) {
+        var items = {};
+        switch (key) {
+          case Constants.AccessSurpriseMeEnabledKey:
+            items[Constants.AccessSurpriseMeEnabledKey] = true;
+          case Constants.AccessLastSurpriseWallpaperChangedDate:
+            items[Constants.AccessLastSurpriseWallpaperChangedDate] =
+                new Date().toDateString();
+        }
+        callback(items);
       },
       set: function(items, callback) {
       }
     },
     onChanged: {
       addListener: function(listener) {
         this.dispatch = listener;
       }
     }
   },
@@ skipping 91 matching lines @@
   };
 
   function uninstallMockXMLHttpRequest() {
     window['XMLHttpRequest'] = originalXMLHttpRequest;
   };
 
   exports.installMockXMLHttpRequest = installMockXMLHttpRequest;
   exports.uninstallMockXMLHttpRequest = uninstallMockXMLHttpRequest;
 
 })(window);