Add an unittest for background/volume_manager.js

chrome/test/data/file_manager/unit_tests/mocks/mock_volume_manager.js

 // Copyright 2013 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.
 
 /**
  * Mock class for VolumeManager.
  * @constructor
  */
 function MockVolumeManager() {
   this.volumeInfoList = new cr.ui.ArrayDataModel([]);
@@ skipping 18 matching lines @@
   return null;
 };
 
 /**
  * Utility function to create a mock VolumeInfo.
  * @param {VolumeType} type Volume type.
  * @param {string} path Volume path.
  * @return {VolumeInfo} Created mock VolumeInfo.
  */
 MockVolumeManager.createMockVolumeInfo = function(type, volumeId) {
-  var fileSystem = new MockFileSystem(volumeId);
+  var fileSystem = new MockFileSystem(volumeId, 'filesystem:' + volumeId);
+  fileSystem.entries['/'] = new MockDirectoryEntry(fileSystem, '/');
 
   var volumeInfo = new VolumeInfo(
       type,
       volumeId,
       fileSystem,
       '',     // error
       '',     // deviceType
       '',     // devicePath
       false,  // isReadonly
       {isCurrentProfile: true, displayName: ''},  // profile
       '');    // label
 
   return volumeInfo;
 };