Ignore fullscreen requests for the current fullscreen element

Ignore fullscreen requests for the current fullscreen element

http://fullscreen.spec.whatwg.org/#dom-element-requestfullscreen

This spec change was made in order to simplify the fix for mis-nested
fullscreen in iframes:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=26568 (comments 9-13)

The requestFullscreen() implementation is not in sync with the spec, but
an early return is equivalent until the "run the remaining steps
asynchronously" bit is implemented.

In order to be affected by this change, one would have to request
fullscreen for the same element twice and do something meaningful in the
fullscreenerror event, which is likely rare.

Note that exitFullscreen() already does nothing if the fullscreen
element stack is empty, so there is some symmetry to this.

TEST=LayoutTests/fullscreen/api/element-request-fullscreen-top.html
TEST=LayoutTests/fullscreen/api/element-request-fullscreen-non-top.html

BUG=403741
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180444

[philipj_slow, 2014-08-16 22:16:42]

PTAL. Baby steps to make regressions easier to bisect.

[philipj_slow, 2014-08-16 22:37:23]

https://codereview.chromium.org/485443002/ is the next step, but it doesn't have
tests yet.

[falken, 2014-08-18 02:58:44]

lgtm

[philipj_slow, 2014-08-18 08:56:15]

rebase

[philipj_slow, 2014-08-18 08:59:18]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2014-08-18 09:00:28]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/philipj@opera.com/482543002/20001

[commit-bot: I haz the power, 2014-08-18 10:13:29]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  mac_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/mac_blink_rel/builds/20159)
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/23175)

[commit-bot: I haz the power, 2014-08-18 10:56:15]

Committed patchset #2 (20001) as 180444