Add UseCounter for <iframe allowfullscreen> exemption for <video>

The video-specific fullscreen API can hopefully be retired soon, but
if it can't it would still be valuable to get rid of this exemption,
which could incentivise people to use the API.

BUG=346236
R=falken@chromium.org

Review URL: https://codereview.chromium.org/399933002

git-svn-id: svn://svn.chromium.org/blink/trunk@178420 bbb929c8-8fbe-4397-9dbb-9b2b20218538