Move MediaGalleries.getMediaFileSystems out of experimental.

chrome/test/data/extensions/api_test/media_galleries/no_galleries/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.
 
-var mediaGalleries = chrome.experimental.mediaGalleries;
+var mediaGalleries = chrome.mediaGalleries;
 
 var mediaFileSystemsListCallback = function(results) {
   chrome.test.assertEq(0, results.length);
 };
 
 chrome.test.runTests([
   function getGalleries() {
     mediaGalleries.getMediaFileSystems(
         chrome.test.callbackPass(mediaFileSystemsListCallback));
   },
 ]);