Settings Router: Restrict navigateToPreviousRoute from going deeper.

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 299 matching lines @@
       if (queryString)
         url += '?' + queryString;
     }
 
     // History serializes the state, so we don't push the actual route object.
     window.history.pushState(currentRoute_.path, '', url);
     setCurrentRoute(route, params);
   };
 
   /**
-   * Navigates to the previous route, but will never exit Settings. If there is
-   * no previous route in the history, navigates to the immediate parent.
+   * Navigates to the previous route if it has an equal or lesser depth.
+   * If there is no previous route in history meeting those requirements,
+   * this navigates to the immediate parent. This will never exit Settings.
    */
   var navigateToPreviousRoute = function() {
     var previousRoute =
         window.history.state &&
         assert(getRouteForPath(/** @type {string} */ (window.history.state)));
 
-    if (previousRoute)
+    if (previousRoute && previousRoute.depth <= currentRoute_.depth)
       window.history.back();
     else
-      navigateTo(settings.getCurrentRoute().parent || Route.BASIC);
+      navigateTo(currentRoute_.parent || Route.BASIC);
   };
 
   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,
     initializeRouteFromUrl: initializeRouteFromUrl,
     getCurrentRoute: getCurrentRoute,
     getQueryParameters: getQueryParameters,
     navigateTo: navigateTo,
     navigateToPreviousRoute: navigateToPreviousRoute,
   };
 });