[Win10 FRE] Fix ARIA attributes to conform to accordion widget.

chrome/browser/resources/welcome/win10/inline.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.
 
 Polymer({
   is: 'welcome-win10-inline',
 
   properties: {
     // Determines if the combined variant should be displayed. The combined
     // variant includes instructions on how to pin Chrome to the taskbar.
@@ skipping 29 matching lines @@
 
   onContinue: function() {
     chrome.send('handleContinue');
   },
 
   onOpenSettings: function() {
     chrome.send('handleSetDefaultBrowser');
   },
 
   onToggle: function() {
-    if (this.isCombined) {
-      var sections = this.shadowRoot.querySelectorAll('.section.expandable');
-      sections.forEach(function(section) {
-        section.classList.toggle('expanded');
-      });
-    }
+    if (!this.isCombined)
+      return;
+    var sections = this.shadowRoot.querySelectorAll('.section.expandable');
+    sections.forEach(function(section) {
+      var isExpanded = section.classList.toggle('expanded');
+      section.querySelector('[role~="button"]').setAttribute(
+          'aria-expanded', isExpanded);
+    });
   }
 });