Use HTMLMediaElement's helpers for the controls fullscreen button

This isolates ExemptIFrameAllowFullScreenRequirement to a single call
site and makes the code more readable.

R=acolwell@chromium.org

Review URL: https://codereview.chromium.org/339903005

git-svn-id: svn://svn.chromium.org/blink/trunk@177045 bbb929c8-8fbe-4397-9dbb-9b2b20218538