Deprecate HTMLVideoElement-specific prefixed fullscreen API

Intent to Deprecate:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/DULRMEUkeJw/9HXn55X1gj4J

Unfortunately Blink doesn't support the unprefixed fullscreen API yet,
so advise authors to use both the unprefixed and prefixed APIs.

webkitSupportsFullscreen was found to be needlessly complicated for what
it does, so a bug to simplify it was filed: http://crbug.com/334985

BUG=334987

Review URL: https://codereview.chromium.org/139313007

git-svn-id: svn://svn.chromium.org/blink/trunk@165288 bbb929c8-8fbe-4397-9dbb-9b2b20218538