Add an initial implementation of the downloads MD settings page.

chrome/browser/resources/settings/downloads_page/demo.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.
 
-/**
- * Wire up the prefs to the a11y page.
- */
-window.onload = function() {
-  var page = document.querySelector('cr-settings-a11y-page');
+// Wire up the prefs to the downloads page.
+window.addEventListener('polymer-ready', function() {
+  var page = document.querySelector('cr-settings-downloads-page');
   page.prefs = document.querySelector('cr-settings-prefs');
-};
+});