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

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178420

[philipj_slow, 2014-07-17 17:28:39]

PTAL

[falken, 2014-07-18 02:05:44]

On 2014/07/17 17:28:39, philipj wrote:
> PTAL

LGTM

[philipj_slow, 2014-07-18 06:01:49]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2014-07-18 06:02:56]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/philipj@opera.com/399933002/1

[commit-bot: I haz the power, 2014-07-18 06:37:52]

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/17641)

[commit-bot: I haz the power, 2014-07-18 07:13:06]

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

[commit-bot: I haz the power, 2014-07-18 07:13:07]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/17645)

[philipj_slow, 2014-07-18 07:36:24]

Committed patchset #1 manually as r178420 (presubmit successful).