Use HTMLMediaElement's helpers for the video fullscreen button

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=177045

[acolwell GONE FROM CHROMIUM, 2014-06-26 15:38:02]

lgtm

[philipj_slow, 2014-06-26 15:42:55]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2014-06-26 15:44:24]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/philipj@opera.com/339903005/1

[commit-bot: I haz the power, 2014-06-26 19:17:22]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/13864)

[commit-bot: I haz the power, 2014-06-26 19:27:24]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-06-26 19:27:24]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/13864)

[philipj_slow, 2014-06-27 02:27:54]

Committed patchset #1 manually as r177045 (presubmit successful).