[push-api] Demo of push messaging at Google I/O

Hi all,

We demoed the push API <https://w3c.github.io/push-api/> and notifications
from a Service Worker
<https://www.mail-archive.com/whatwg@lists.whatwg.org/msg37350.html> at
Google I/O as part of broader talk about Service Workers (also covering
offline usage, and the proposed background sync API
<https://github.com/slightlyoff/BackgroundSync>).

If you're interested, you can watch at
https://www.google.com/events/io/schedule/session/f9a35989-07b6-e311-8491-00155d5066d7
or
skip to the part about Push 25 minutes and 40 seconds in
<https://www.youtube.com/watch?v=_yy0CDLnhMA#t=1538>.

The demo was an early prototype running on Chrome for Android; the basic
skeleton is checked in behind a flag, but we're still working on landing
various patches (not to mention productionizing it).

John