Settings Router Refactor: Update subpage-back button to use history back sometimes.

chrome/browser/resources/settings/route.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.
 
 cr.define('settings', function() {
   /**
    * Class for navigable routes. May only be instantiated within this file.
    * @constructor
    * @param {string} path
    * @private
@@ skipping 340 matching lines @@
       return;
     }
 
     var url = route.path;
     if (opt_dynamicParameters) {
       var queryString = opt_dynamicParameters.toString();
       if (queryString)
         url += '?' + queryString;
     }
 
+    // History serializes the state, so we don't push the actual route object.
+    var previousRoutePath = currentRoute_.path;
     setCurrentRoute(route, params);
-    window.history.pushState(undefined, '', url);
+    window.history.pushState(previousRoutePath, '', url);
   };
 
   window.addEventListener('popstate', function(event) {
     // On pop state, do not push the state onto the window.history again.
     setCurrentRoute(getRouteForPath(window.location.pathname) || Route.BASIC,
                     new URLSearchParams(window.location.search));
   });
 
   return {
     Route: Route,
     RouteObserverBehavior: RouteObserverBehavior,
     getRouteForPath: getRouteForPath,
     getCurrentRoute: getCurrentRoute,
     getQueryParameters: getQueryParameters,
     navigateTo: navigateTo,
   };
 });