Introduce FullscreenElementStack::RequestType

Introduce FullscreenElementStack::RequestType

This replaces flags + FullScreenCheckType, for a few reasons:

 1. An UnprefixedRequest will be added, for which neither
    ALLOW_KEYBOARD_INPUT nor FullScreenCheckType is possible. Not all
    previously possible combinations are possible, either.

 2. An enum is clearer than flags at the call site. In particular,
    0 was passed to HTMLMediaElement::enterFullscreen() which would
    disallow keyboard input, which was not obivous and possibly not
    even the original intention. The behavior was preserved but should
    probably be changed to allow keyboard input in a future patch.

 3. LEGACY_MOZILLA_REQUEST as a public part of Element.h is weird.

BUG=383813
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=177061

[philipj_slow, 2014-06-26 15:08:09]

PTAL. Note that this depends on https://codereview.chromium.org/339903005/ to
land first, so no try bots yet. LayoutTests/fullscreen/ and LayoutTests/media/
pass locally, though.

[davve, 2014-06-26 15:11:25]

Non-owner LGTM

[falken, 2014-06-27 01:00:51]

the code is much easier to understand now! lgtm

[philipj_slow, 2014-06-27 02:29:24]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2014-06-27 02:30:07]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/philipj@opera.com/356933002/1

[commit-bot: I haz the power, 2014-06-27 04:16:08]

Change committed as 177061