MD Settings: Add mechanism to restore focus after subpage exiting.

chrome/browser/resources/settings/search_page/search_page.html

 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/md_select_css.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/neon-animation/neon-animatable.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="../controls/extension_controlled_indicator.html">
 <link rel="import" href="../controls/settings_toggle_button.html">
 <link rel="import" href="../icons.html">
 <link rel="import" href="../i18n_setup.html">
@@ skipping 15 matching lines @@
       }
 
       iron-icon {
         -webkit-padding-end: 16px;
       }
 
       .indented {
         -webkit-margin-start: var(--settings-indent-width);
       }
     </style>
-    <settings-animated-pages id="pages" section="search">
+    <settings-animated-pages id="pages" section="search"
+        focus-config="[[focusConfig_]]">
       <neon-animatable route-path="default">
         <!-- Omnibox search engine -->
         <div class="settings-box first block">
           <div id="search-wrapper">
             <p class="start">$i18nRaw{searchExplanation}</p>
             <div>
               <select class="md-select" on-change="onChange_"
                   disabled$="[[
                       prefs.default_search_provider.enabled.extensionId]]">
                 <template is="dom-repeat" items="[[searchEngines_]]">
@@ skipping 86 matching lines @@
         <settings-subpage
             associated-control="[[$$('#subpage-trigger')]]"
             page-title="$i18n{searchEnginesManage}">
           <settings-search-engines-page></settings-search-engines-page>
         </settings-subpage>
       </template>
     </settings-animated-pages>
   </template>
   <script src="search_page.js"></script>
 </dom-module>