[Gallery app] Make the background page use BackgroundBase class

ui/file_manager/integration_tests/gallery/test_util.js

 // Copyright 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';
 
 /**
  * Gets file entries just under the volume.
  *
  * @param {VolumeManagerCommon.VolumeType} volumeType Volume type.
@@ skipping 101 matching lines @@
  */
 function launchWithTestEntries(
     testVolumeName, volumeType, entries, opt_selected) {
   var entriesPromise = addEntries([testVolumeName], entries).then(function() {
     var selectedEntries = opt_selected || entries;
     return getFilesUnderVolume(
         volumeType,
         selectedEntries.map(function(entry) { return entry.nameText; }));
   });
 
-  return launch(entriesPromise).then(function() {
-    var launchedPromise = Promise.all([appWindowPromise, entriesPromise]);
-    return launchedPromise.then(function(results) {
-      return {appWindow: results[0], entries: results[1]};
+  return entriesPromise.then(function(entries) {
+    return window.initializePromise.then(function() {
+      var urls = util.entriesToURLs(entries);
+      var launchedPromise = openGalleryWindow(urls, false);
+      return launchedPromise.then(function(appWindow) {
+        return {appWindow: appWindow, entries: entries};
+      });
     });
   });
 }
 
 /**
  * Waits until the expected image is shown.
  *
  * @param {document} document Document.
  * @param {number} width Expected width of the image.
  * @param {number} height Expected height of the image.
@@ skipping 11 matching lines @@
       height: fullResCanvas && fullResCanvas.height,
       name: nameBox && nameBox.value
     };
     if (!chrome.test.checkDeepEq(expected, actual)) {
       return pending('Slide mode state, expected is %j, actual is %j.',
                      expected, actual);
     }
     return actual;
   });
 }