Settings Rewrite: Restore id of default id to 'main' from ''.

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

 <link rel="import" href="chrome://resources/polymer/v1_0/polymer/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://md-settings/checkbox/checkbox.html">
 <link rel="import" href="chrome://md-settings/search_engines_page/search_engines_page.html">
 <link rel="import" href="chrome://md-settings/settings_page/settings_animated_pages.html">
 <link rel="import" href="chrome://md-settings/settings_page/settings_subheader.html">
 
 <dom-module id="cr-settings-search-page">
   <link rel="import" type="css"
       href="chrome://md-settings/settings_page/settings_page.css">
   <link rel="import" type="css" href="search_page.css">
   <template>
     <settings-animated-pages id="pages" current-route="{{currentRoute}}"
         section="search">
-      <neon-animatable id="">
+      <neon-animatable id="main">
         <p i18n-content="searchExplanation"></p>
 
         <div class="search-engines">
           <select id="searchEnginesMenu" on-change="defaultEngineGuidChanged_">
             <template is="dom-repeat" items="[[searchEngines]]">
               <option value="[[item.guid]]">{{item.name}}</option>
             </template>
           </select>
 
           <paper-button class="search-engines"
@@ skipping 14 matching lines @@
       <neon-animatable id="search-engines-advanced">
         <!-- This sub-subpage is purely a placeholder to provide an example. -->
         <settings-subheader i18n-values="page-title:searchEnginesLabel">
         </settings-subheader>
         <p i18n-content="searchExplanation"></p>
       </neon-animatable>
     </settings-animated-pages>
   </template>
   <script src="search_page.js"></script>
 </dom-module>