Remove the fullscreen element stack exemption for webkitRequestFullScreen()

Remove the fullscreen element stack exemption for webkitRequestFullScreen()

This exemption allowed the fullscreen element stack to get into a state
not otherwise possible, namely with elements in any order as opposed to
in tree order. It also seemed possible to enter fullscreen in a sibling
iframe, leading to a (cross-document) set of fullscreen elements forming
a tree instead of a simple chain, a case without any test coverage.

Removing the exemption means that a webkitfullscreenerror event will be
fired if one is already in fullscreen and requests fullscreen for an
element other than a descendant of the current fullscreen element.

The use counter was intended to tell whether any content depends on this
or not, but data will not be available for a long time yet. In order to
be affected one needs to, in fullscreen, have a currently visible button
request fullscreen for an element currently not visible, which seems
far-fetched, so site compat is probably not a problem.

The use counter for webkitRequestFullScreen() provides an upper bound:
http://www.chromestatus.com/metrics/feature/timeline/popularity/177

BUG=383813
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180243

[philipj_slow, 2014-08-12 21:35:18]

PTAL. There's complexity here I'd rather not have to think about, so I think we
should try removal. It's not much to revert in case my hunch is wrong.

[falken, 2014-08-13 04:39:50]

looks good

Is it worth adding a test case for erroring when you try to fullscreen a
non-descendant of the fullscreened element with the prefixed mozilla API?

[philipj_slow, 2014-08-13 08:40:41]

As it turns out we already had good coverage with full-screen-twice-newapi.html,
so rather than changing the order of full-screen-twice.html I'll verify that the
second request fails. full-screen-css.html will implicitly cover the successful
case, and full-screen-twice-newapi.html does it explicitly.

[philipj_slow, 2014-08-13 08:46:52]

tweak test

[philipj_slow, 2014-08-13 10:58:04]

spec typo

[falken, 2014-08-14 02:15:30]

lgtm

[philipj_slow, 2014-08-14 05:20:45]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2014-08-14 05:21:35]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/philipj@opera.com/458223003/40001

[commit-bot: I haz the power, 2014-08-14 06:24:29]

Committed patchset #3 (40001) as 180243