Adding strict mode for bookmark_manager file for better error checking.

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

 // Copyright (c) 2011 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.
 
 
 cr.define('bmm', function() {
+  'use strict';
+
   /**
    * The id of the bookmark root.
    * @type {string}
    * @const
    */
   var ROOT_ID = '0';
 
   /** @const */ var Tree = cr.ui.Tree;
   /** @const */ var TreeItem = cr.ui.TreeItem;
   /** @const */ var localStorage = window.localStorage;
@@ skipping 295 matching lines @@
   };
 
   return {
     BookmarkTree: BookmarkTree,
     BookmarkTreeItem: BookmarkTreeItem,
     treeLookup: treeLookup,
     tree: /** @type {Element} */(null),  // Set when decorated.
     ROOT_ID: ROOT_ID
   };
 });