MD Settings: adjust iron-list focus row behaviors.

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

 <link rel="import" href="chrome://resources/cr_elements/cr_action_menu/cr_action_menu.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_lazy_render/cr_lazy_render.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/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="startup_urls_page_browser_proxy.html">
+<link rel="import" href="../focus_row_behavior.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="list-item" focus-row-container>
       <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>
       <template is="dom-if" if="[[editable]]">

[hcarmona, 2017/03/16 21:31:54]

What happens if nothing is actionable? Where does focus go?

[scottchen, 2017/03/17 21:26:58]

Discussed offline and found no case of a row that has absolutely no focusable
control. I've adjusted the code to fail with an assert() instead of null
pointer, so that it's more obvious the assumption was intentional.

-        <paper-icon-button id="dots" icon="cr:more-vert"
-            tabindex$="[[tabindex]]" on-tap="onDotsTap_"
-            title="$i18n{moreActions}">
+        <paper-icon-button id="dots" icon="cr:more-vert" on-tap="onDotsTap_"
+            title="$i18n{moreActions}" focus-row-control type="menu">
         </paper-icon-button>
         <template is="cr-lazy-render" id="menu">
           <dialog is="cr-action-menu">
             <button class="dropdown-item" role="option" on-tap="onEditTap_">
               $i18n{edit}
             </button>
             <button class="dropdown-item" role="option" id="remove"
                 on-tap="onRemoveTap_">
               $i18n{onStartupRemove}
             </button>
           </dialog>
         </template>
       </template>
     </div>
   </template>
   <script src="startup_url_entry.js"></script>
 </dom-module>