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

chrome/browser/resources/settings/settings_page/settings_animated_pages.js

 // Copyright 2015 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-animated-pages' is a container for a page and animated subpages.
  * It provides a set of common behaviors and animations.
  *
  * Example:
@@ skipping 58 matching lines @@
       if (newRoute.subpage.length > oldRoute.subpage.length) {
         this.$.animatedPages.exitAnimation = 'slide-left-animation';
         this.$.animatedPages.entryAnimation = 'slide-from-right-animation';
       } else {
         this.$.animatedPages.exitAnimation = 'slide-right-animation';
         this.$.animatedPages.entryAnimation = 'slide-from-left-animation';
       }
     }
 
     this.$.animatedPages.selected =
-        newRouteIsSubpage ? newRoute.subpage.slice(-1)[0] : '';
+        newRouteIsSubpage ? newRoute.subpage.slice(-1)[0] : 'main';
   },
 
   /**
    * Buttons in this pageset should use this method to transition to subpages.
    */
   setSubpageChain: function(subpage) {
     this.currentRoute = {
       page: this.currentRoute.page,
       section: subpage.length > 0 ? this.section : '',
       subpage: subpage,
     };
   },
 });