MD Settings: Fix rendering of popup menus inside iron-lists.

chrome/browser/resources/settings/on_startup_page/startup_url_entry.js

 // Copyright 2016 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.
 
 /**
  * @fileoverview settings-startup-url-entry represents a UI component that
  * displayes a URL that is loaded during startup. It includes a menu that allows
  * the user to edit/remove the entry.
  */
 
 cr.exportPath('settings');
 
 /**
  * The name of the event fired from this element when the "Edit" option is
  * tapped.
  * @const {string}
  */
 settings.EDIT_STARTUP_URL_EVENT = 'edit-startup-url';
 
 Polymer({
   is: 'settings-startup-url-entry',
 
+  behaviors: [ListItemBehavior],
+
   properties: {
     /** @type {!StartupPageInfo} */
     model: Object,
   },
 
   /**
    * @param {string} url Location of an image to get a set of icons for.
    * @return {string} A set of icon URLs.
    * @private
    */
   getIconSet_: function(url) {
     return cr.icon.getFavicon(url);
   },
 
   /** @private */
   onRemoveTap_: function() {
     this.$$('iron-dropdown').close();
     settings.StartupUrlsPageBrowserProxyImpl.getInstance().removeStartupPage(
         this.model.modelIndex);
   },
 
   /** @private */
   onEditTap_: function() {
     this.$$('iron-dropdown').close();
     this.fire(settings.EDIT_STARTUP_URL_EVENT, this.model);
   },
 });