Prevent a null crash when window.history is called during detach.

Prevent a null crash when window.history is called during detach.

In certain rare cases, history.{push,replace}State are called when during
plugin detach. In this case, it's possible for the state modification to be
requested on a frame that is half-way detached. The Document is still live,
but the DocumentLoader is gone. Exit early in this case.

BUG=356424
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170219

[Nate Chapin, 2014-03-27 18:40:12]

The repro involves plugins doing crazy things synchronously on teardown, thus
sapping all motivation to create a repro.

[abarth-chromium, 2014-03-27 21:06:46]

plugins -> cry

LGTM

[Nate Chapin, 2014-03-27 21:08:05]

The CQ bit was checked by japhet@chromium.org

[commit-bot: I haz the power, 2014-03-27 21:08:27]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/japhet@chromium.org/215183002/1

[commit-bot: I haz the power, 2014-03-27 21:19:41]

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

[commit-bot: I haz the power, 2014-03-27 21:19:41]

Try jobs failed on following builders:
  tryserver.blink on win_blink_rel

[Nate Chapin, 2014-03-27 21:20:40]

The CQ bit was checked by japhet@chromium.org

[commit-bot: I haz the power, 2014-03-27 21:20:46]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/japhet@chromium.org/215183002/1

[commit-bot: I haz the power, 2014-03-27 22:33:04]

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

[commit-bot: I haz the power, 2014-03-27 22:33:04]

Commit queue rejected this change because the description was changed
between the time the change entered the commit queue and the time it
was ready to commit. You can safely check the commit box again.

[Nate Chapin, 2014-03-27 23:02:15]

The CQ bit was checked by japhet@chromium.org

[commit-bot: I haz the power, 2014-03-27 23:02:18]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/japhet@chromium.org/215183002/1

[commit-bot: I haz the power, 2014-03-27 23:27:08]

Change committed as 170219