Chromecast devices supported through Google Cast Extension

The demo may now send the video to Chromecast devices, provided the page runs
in Chrome with the Google Cast Extension installed:
https://chrome.google.com/webstore/detail/google-cast/boadgeojelhgndaghljhdicfkmllpafd?hl=en

The page loads and initializes the Google Cast API on startup.

When the user hits the "share" button, the page:
1. tries to create a session on a Chromecast device if the API reports there
are such devices available. Chrome will typically prompt the user to select
a Chromecast device in the list of devices available.
2. falls back to previous behavior if there are no Chromecast devices around
or if the user declines to use a Chromecast device.

Notes:
- if the user declines to use a Chromecast device, the call to
"window.open" occurs in a fallback function and Chrome will block it by
default. Pop-ups need to be allowed for the fallback to work in that case.
- the video only pauses on the initial page when it is ready to play on the
Chromecast device. From a UX perspective, it might be better to pause the
video right away though.
- sending the video to a nearby Chromecast device may take a few seconds,
the code does not implement any sort of protection mechanism: it must be
relatively easy to break the demo if you click around before an action is
over.
- the video content-type is hardcoded for the time being. Chromecast needs it
to play the video, it seems. The content-type could be read from the video
element instead.
- the page keeps the Chromecast session open even if the user stops sharing
the video. The session can be terminated through the extension menu. The
page could perhaps close the session permanently, not sure which approach is
better.