'use strict' in mock_folder_shortcut_data_model.js

If we can use ...args @constructor, use strict as well.

Bug: 910991
Change-Id: I6181423d7df7ab1a376650c657bdb3d8b0636f3c
Reviewed-on: https://chromium-review.googlesource.com/c/1358323Reviewed-by: Stuart Langley <slangley@chromium.org>
Commit-Queue: Stuart Langley <slangley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613085}

ui/file_manager/file_manager/foreground/js/mock_folder_shortcut_data_model.js

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+'use strict';
+
 /**
  * Mock FolderShortcutDataModel.
  * @param {!Array<MockEntry>} args Array of shortcuts.