Files.app: Add the MockDirectoryEntry class.

chrome/test/data/file_manager/unit_tests/mocks/mock_file_entry.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 FileEntry.
- *
- * @param {string} volumeId Id of the volume containing the entry.
- * @param {string} fullPath Full path for the entry.
- * @constructor
- */
-function MockFileEntry(volumeId, fullPath) {
-  this.volumeId = volumeId;
-  this.fullPath = fullPath;
-}
-
-/**
- * Returns fake URL.
- *
- * @return {string} Fake URL.
- */
-MockFileEntry.prototype.toURL = function() {
-  return 'filesystem:' + this.volumeId + this.fullPath;
-};