Set up serving infrastructure for c/b/r/settings.

chrome/browser/resources/settings/a11y_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() {

[michaelpg, 2015/03/06 20:25:59]

I believe it's more "correct" to use window.addEventListener('polymer-ready',
...) to ensure the elements have been registered.

[Jeremy Klein, 2015/03/06 21:33:56]

This isn't really necessary in this case because the script here doesn't depend
on any of the Polymer elements being upgraded, just that the DOM is loaded.
Also, do you know if polymer-ready is guaranteed to fire *after* window.onload?
I know that it will probably almost always be true, but I'm not sure there's a
guarantee there. If we got into a state where the polymer-ready fired before
window.onload, we'd still hit errors in here.

I'll change it if you feel really strongly about it, but I think for a demo that
isn't actually relying on polymer elements being fully registered, this doesn't
matter much.

[michaelpg, 2015/03/06 22:33:49]

Data binding seems to work, but apparently it's not going through all the
motions. If the element hasn't been upgraded, setting page.prefs just sets that
property (which is all the page needs for data binding to start kicking in).

Once the element has been upgraded, setting page.prefs would cause changed
watchers to be called, updates page.prefs_, and whatever else Polymer does under
the hood. If the attribute were reflected, the reflection wouldn't happen. So
all of that doesn't happen if the element hasn't been upgraded yet. I don't know
whether there's a guarantee in terms of what will work and what won't.
I can't find that guarantee. But could polymer-ready be fired without the DOM?
My understanding is it can only be fired once all polymer-element declarations
have been executed. Can it ensure that before it's queried the entire DOM tree?
I agree it doesn't matter much, if it's committed and it works. I thought it's
worth pointing out, since when I reviewed this initially I figured it would work
then too. Just trying to be thorough.

+  var page = document.querySelector('cr-settings-a11y-page');
+  page.prefs = document.querySelector('cr-settings-prefs');
+};