Restore the HTMLVideoElement-specific prefixed fullscreen API behind a flag

The removal has caused more trouble than expected on Android, because
usage is higher there and the UseCounter data from Windows was used in
the "Intent to Remove" thread.

Restore the API behind a flag so that it can more easily be turned on
and off. It's off by default, but likely it will have to be enabled on
at least the upcoming M35 branch.

BUG=346236

Review URL: https://codereview.chromium.org/213193010

git-svn-id: svn://svn.chromium.org/blink/trunk@170303 bbb929c8-8fbe-4397-9dbb-9b2b20218538