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

chrome/browser/resources/settings/settings_page/settings_subpage.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.
 
 /**
  * @fileoverview
  * 'settings-subpage' shows a subpage beneath a subheader. The header contains
  * the subpage title and a back icon.
  */
 
@@ skipping 25 matching lines @@
      * @type {?HTMLElement}
      */
     associatedControl: {
       type: Object,
       value: null,
     },
   },
 
   /** @private */
   onTapBack_: function() {
-    settings.navigateTo(assert(settings.getCurrentRoute().parent));
+    var previousRoute =
+        window.history.state &&
+        assert(settings.getRouteForPath(
+            /** @type {string} */ (window.history.state)));
+
+    if (previousRoute && previousRoute.contains(settings.getCurrentRoute()))
+      window.history.back();
+    else
+      settings.navigateTo(assert(settings.getCurrentRoute().parent));
   },
 
   /** @private */
   onSearchChanged_: function(e) {
     this.searchTerm = e.detail;
   },
 });