Add extension permissions for Media Gallery API.

chrome/test/data/extensions/api_test/media_gallery/basic/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 mediaFileSystemsListCallback = function(results) {
   // There should be a "Pictures" directory on all desktop platforms.
   var expectedFileSystems = 1;
   // But not on Android and ChromeOS.
   if (/Android/.test(navigator.userAgent) || /CrOS/.test(navigator.userAgent)) {
@@ skipping 28 matching lines @@
     var result = mediaGalleries.extractEmbeddedThumbnails({});
     chrome.test.assertEq(null, result);
     chrome.test.succeed()
   },
 
   function assembleMediaFile() {
     mediaGalleries.assembleMediaFile(
         {}, {}, chrome.test.callbackPass(nullCallback));
   },
 ]);