Media Galleries API: hookup the AllGalleries permission and add tests.

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.mediaGalleries;
+var mediaFileSystemsListCallback = function(results) {
+  chrome.test.assertEq(0, results.length);
+};
+
+chrome.test.runTests([
+  function getGalleries() {
+    mediaGalleries.getMediaFileSystems(
+        chrome.test.callbackPass(mediaFileSystemsListCallback));
+  },
+]);