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

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

 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-dropdown/iron-dropdown.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
+<link rel="import" href="/list_item_behavior.html">
 <link rel="import" href="/on_startup_page/startup_urls_page_browser_proxy.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-startup-url-entry">
   <template>
     <style include="settings-shared">
       .hide-overflow {
         overflow: hidden;
       }
     </style>
     <div class="list-item">
       <div class="favicon-image"
           style="background-image: [[getIconSet_(model.url)]]">
       </div>
       <div class="middle hide-overflow">
         <div class="text-elide">[[model.title]]</div>
         <div class="text-elide secondary">[[model.url]]</div>
       </div>
       <paper-icon-button id="dots" icon="cr:more-vert" toggles
-          active="{{menuOpened}}" tabindex$="[[tabindex]]">
+          active="{{menuOpened}}" tabindex$="[[tabindex]]"
+          on-tap="ensureMaxZIndex">
       </paper-icon-button>
       <template is="dom-if" if="[[menuOpened]]">
         <iron-dropdown vertical-align="auto" horizontal-align="right"
             opened="{{menuOpened}}">
           <div class="dropdown-content">
             <button class="dropdown-item" role="option" on-tap="onEditTap_">
               $i18n{onStartupEdit}
             </button>
             <button class="dropdown-item" role="option" id="remove"
                 on-tap="onRemoveTap_">
               $i18n{onStartupRemove}
             </button>
           </div>
         </iron-dropdown>
       </template>
     </div>
   </template>
   <script src="startup_url_entry.js"></script>
 </dom-module>