Add tests for sync custom wallpaper feature

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: '*#*@#&'
 };
@@ skipping 48 matching lines @@
   },
   alarms: {
     onAlarm: {
       addListener: function(listener) {
       }
     }
   },
   wallpaperPrivate: {
     getStrings: function(callback) {
       callback({isExperimental: false});
+    },
+    setCustomWallpaper: function(data, layout, isGenerateThumbnail, fileName,
+                                 callback) {
     }
+  },
+  runtime: {
+    lastError: null
   }
 };
 
 (function (exports) {
   var originalXMLHttpRequest = window.XMLHttpRequest;
 
   // Mock XMLHttpRequest. It dispatches a 'load' event immediately with status
   // equals to 200 in function |send|.
   function MockXMLHttpRequest() {
   }
@@ skipping 46 matching lines @@
   };
 
   function uninstallMockXMLHttpRequest() {
     window['XMLHttpRequest'] = originalXMLHttpRequest;
   };
 
   exports.installMockXMLHttpRequest = installMockXMLHttpRequest;
   exports.uninstallMockXMLHttpRequest = uninstallMockXMLHttpRequest;
 
 })(window);