Add the browser test for the new gallery.

chrome/test/data/extensions/api_test/file_manager_browsertest/thumbnails.js

-// Copyright (c) 2014 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.
-
-'use strict';
-
-/**
- * Tests if a thumbnail for the selected item shows up in the preview panel.
- * This thumbnail is fetched via the image loader.
- */
-testcase.thumbnailsDownloads = function() {
-  var appId;
-  StepsRunner.run([
-    function() {
-      setupAndWaitUntilReady(null, RootPath.DOWNLOADS, this.next);
-    },
-    // Select the image.
-    function(inAppId) {
-      appId = inAppId;
-      callRemoteTestUtil('selectFile',
-                         appId,
-                         ['My Desktop Background.png'],
-                         this.next);
-    },
-    // Wait until the thumbnail shows up.
-    function(result) {
-      chrome.test.assertTrue(result);
-      waitForElement(appId, '.preview-thumbnails .img-container img').
-          then(this.next);
-    },
-    // Verify the thumbnail.
-    function(element) {
-      chrome.test.assertTrue(element.attributes.src.indexOf(
-          'data:image/jpeg') === 0);
-      checkIfNoErrorsOccured(this.next);
-    }
-  ]);
-};