Fix bookmark manager regression by allowing cr.ui.Menu#findMenuItem_ to return null.

ui/webui/resources/js/cr/ui/menu.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.
 
 cr.define('cr.ui', function() {
 
   /** @const */ var MenuItem = cr.ui.MenuItem;
 
   /**
    * Creates a new menu element. Menu dispatches all commands on the element it
@@ skipping 78 matching lines @@
      * Walks up the ancestors of |el| until a menu item belonging to this menu
      * is found.
      * @param {Element} el The element to start searching from.
      * @return {cr.ui.MenuItem} The found menu item or null.
      * @private
      */
     findMenuItem_: function(el) {
       while (el && el.parentNode != this) {
         el = el.parentNode;
       }
-      return assertInstanceof(el, MenuItem);
+      return el ? assertInstanceof(el, MenuItem) : null;
     },
 
     /**
      * Handles mouseover events and selects the hovered item.
      * @param {Event} e The mouseover event.
      * @private
      */
     handleMouseOver_: function(e) {
       var overItem = this.findMenuItem_(e.target);
       this.selectedItem = overItem;
@@ skipping 160 matching lines @@
    * @type {number}
    */
   cr.defineProperty(Menu, 'selectedIndex', cr.PropertyKind.JS,
       selectedIndexChanged);
 
   // Export
   return {
     Menu: Menu
   };
 });