Settings Rewrite: Make subpage scrolling work.

chrome/browser/resources/settings/settings_main/settings_main.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
  * 'cr-settings-main' displays the selected settings page.
  *
  * Example:
  *
@@ skipping 30 matching lines @@
     /**
      * Container that determines the sizing of expanded sections.
      */
     expandContainer: {
       type: Object,
     },
   },
 
   /** @override */
   ready: function() {
-    this.expandContainer = this.$.mainContainer;
+    this.expandContainer = this.$.pageContainer;
   },
 
   /** @private */
   getSelectedPage_: function(currentRoute) {
     return currentRoute.page || 'basic';
   },
 });