MD Settings: Fix Back navigation from section route to BASIC route.

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

 // Copyright 2016 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.
 
 /**
  * Responds to route changes by expanding, collapsing, or scrolling to sections
  * on the page. Expanded sections take up the full height of the container. At
  * most one section should be expanded at any given time.
  * @polymerBehavior MainPageBehavior
  */
@@ skipping 12 matching lines @@
   /**
    * @param {!settings.Route} newRoute
    * @param {settings.Route} oldRoute
    */
   currentRouteChanged: function(newRoute, oldRoute) {
     // Scroll to the section except for back/forward. Also scroll for any
     // back/forward navigations that are in-page.
     var oldRouteWasSection =
         !!oldRoute && !!oldRoute.parent && !!oldRoute.section &&
         oldRoute.parent.section != oldRoute.section;
+
+    if (oldRouteWasSection && newRoute == settings.Route.BASIC) {
+      this.scroller.scrollTop = 0;
+      return;
+    }
+
     var scrollToSection =
         !settings.lastRouteChangeWasPopstate() || oldRouteWasSection;
 
     // If this is the first route, or the page was hidden, allow the page to
     // render before expanding the section.
     if (!oldRoute && newRoute.contains(settings.getCurrentRoute()) ||
         this.scrollHeight == 0) {
       setTimeout(this.tryTransitionToSection_.bind(this, scrollToSection));
     } else {
       this.tryTransitionToSection_(scrollToSection);
@@ skipping 225 matching lines @@
     return /** @type {?SettingsSectionElement} */(
         this.$$('settings-section[section="' + section + '"]'));
   },
 };
 
 /** @polymerBehavior */
 var MainPageBehavior = [
   settings.RouteObserverBehavior,
   MainPageBehaviorImpl,
 ];