[MD settings] refine routing function

chrome/browser/resources/settings/settings_page/settings_animated_pages.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
  * 'settings-animated-pages' is a container for a page and animated subpages.
  * It provides a set of common behaviors and animations.
  *
  * Example:
@@ skipping 128 matching lines @@
 
     // Render synchronously so neon-animated-pages can select the subpage.
     template.if = true;
     template.render();
   },
 
   /**
    * Buttons in this pageset should use this method to transition to subpages.
    */
   setSubpageChain: function(subpage) {
+    var node = window.event.currentTarget;
+    var page;
+    while (node) {
+      if (node.dataset && node.dataset.page)
+        page = node.dataset.page;
+      // A shadow root has a |host| rather than a |parentNode|.
+      node = node.host || node.parentNode;
+    }
     this.currentRoute = {
-      page: this.currentRoute.page,
+      page: page,
       section: subpage.length > 0 ? this.section : '',
       subpage: subpage,
     };
   },
 });