Adding strict mode for bookmark_manager file for better error checking.

chrome/browser/resources/bookmark_manager/js/bmm/bookmark_list.js

 // Copyright (c) 2012 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.
 
 // TODO(arv): Now that this is driven by a data model, implement a data model
 //            that handles the loading and the events from the bookmark backend.
 
+'use strict';
+
 /**
  * @typedef {{childIds: Array<string>}}
  *
  * @see chrome/common/extensions/api/bookmarks.json
  */
 var ReorderInfo;
 
 /**
  * @typedef {{parentId: string,
  *            index: number,
@@ skipping 608 matching lines @@
         }
       }
     }
   };
 
   return {
     BookmarkList: BookmarkList,
     list: /** @type {Element} */(null),  // Set when decorated.
   };
 });