Add UseCounter for the Mozilla-style fullscreen error handling exemptions

webkitRequestFullScreen skips some checks that webkitRequestFullscreen
and other modes have.

It is unlikely that the prefixed fullscreen APIs can be removed any time
soon, so trying to reduce the differences between them is worthwhile.

Review URL: https://codereview.chromium.org/363493002

git-svn-id: svn://svn.chromium.org/blink/trunk@177284 bbb929c8-8fbe-4397-9dbb-9b2b20218538