Media Gallery: Implement a basic version of GetMediaFileSystems().

chrome/test/data/extensions/api_test/media_gallery/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 emptyListCallback = function(results) {
-  chrome.test.assertEq(results, []);
+var mediaFileSystemsListCallback = function(results) {
+  // There should be a "Pictures" directory on all desktop platforms.
+  chrome.test.assertEq(1, results.length);
 };
 var nullCallback = function(result) {
-  chrome.test.assertEq(result, null);
+  chrome.test.assertEq(null, result);
 };
 
 chrome.test.runTests([
   function getGalleries() {
     mediaGalleries.getMediaFileSystems(
-        chrome.test.callbackPass(emptyListCallback));
+        chrome.test.callbackPass(mediaFileSystemsListCallback));
   },
 
   function openMediaGalleryManager() {
     // Just confirm that the function exists.
     chrome.test.assertTrue(mediaGalleries.openMediaGalleryManager !== null);
     chrome.test.succeed()
   },
 
   function extractEmbeddedThumbnails() {
     var result = mediaGalleries.extractEmbeddedThumbnails({});
-    chrome.test.assertEq(result, null);
+    chrome.test.assertEq(null, result);
     chrome.test.succeed()
   },
 
   function assembleMediaFile() {
     mediaGalleries.assembleMediaFile(
         {}, {}, chrome.test.callbackPass(nullCallback));
   },
 ]);