[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 = event.currentTarget;

[dpapad, 2016/05/27 00:44:22]

Where is |event| coming from?

[dschuyler, 2016/05/27 01:30:49]

I commented that it's a global, but there
is still the question of whether this should
be plumbed through from the caller instead
of using the global. (Note that we're not
creating the global for this, it already 
exists).

Done.

+    var page;
+    while (node) {
+      if (node.dataset && node.dataset.page)
+        page = node.dataset.page;
+      node = node.host || node.parentNode;

[dpapad, 2016/05/27 00:44:22]

What is the expected type(s) of |node|? I recognize "parentNode" as
https://developer.mozilla.org/en-US/docs/Web/API/Node/parentNode, but don't
recognize "host". Could you add @type annotatino or a comment?

[dschuyler, 2016/05/27 01:30:49]

Done.

+    }
     this.currentRoute = {
-      page: this.currentRoute.page,
+      page: page,
       section: subpage.length > 0 ? this.section : '',
       subpage: subpage,
     };
   },
 });