Add autotestPrivate.getVisibleNotifications API

chrome/test/data/extensions/api_test/autotest_private/test.js

 // Copyright (c) 2012 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.
 
 chrome.test.runTests([
   // logout/restart/shutdown don't do anything as we don't want to kill the
   // browser with these tests.
   function logout() {
     chrome.autotestPrivate.logout();
     chrome.test.succeed();
@@ skipping 86 matching lines @@
     chrome.autotestPrivate.setNaturalScroll(true);
     chrome.test.succeed();
   },
   function setMouseSensitivity() {
     chrome.autotestPrivate.setMouseSensitivity(3);
     chrome.test.succeed();
   },
   function setPrimaryButtonRight() {
     chrome.autotestPrivate.setPrimaryButtonRight(false);
     chrome.test.succeed();
+  },
+  function getVisibleNotifications() {
+    chrome.autotestPrivate.getVisibleNotifications(function(){});
+    chrome.test.succeed();
   }
 ]);