[aw] Fix crash when "misbehave" app exits fullscreen.

[aw] Fix crash when "misbehave" app exits fullscreen.

exitFullScreen() might be call without a prior call to
enterFullScreen() if a "misbehave" app overrides onShowCustomView
but does not add the custom view to the window. This fixes a
crash by returning early from exitFullScreen in that case.

BUG=408133
Committed: https://crrev.com/c0e9f01149baa440a7e4f19ba2705f17017a7b43
Cr-Commit-Position: refs/heads/master@{#292165}

[Ignacio Solla, 2014-08-27 15:22:38]

igsolla@chromium.org changed reviewers:
+ benm@chromium.org

[benm (inactive), 2014-08-27 15:30:02]

lgtm; however will need a crbug + merge request to M38 please.

[Ignacio Solla, 2014-08-27 15:33:45]

The CQ bit was checked by igsolla@chromium.org

[commit-bot: I haz the power, 2014-08-27 15:35:01]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/igsolla@chromium.org/501093007/1

[commit-bot: I haz the power, 2014-08-27 16:21:25]

Committed patchset #1 (id:1) as 6fde5e90aacdd861d913cb73b1002c0f98ebd575

[commit-bot: I haz the power, 2014-09-10 02:52:08]

Patchset 1 (id:??) landed as
https://crrev.com/c0e9f01149baa440a7e4f19ba2705f17017a7b43
Cr-Commit-Position: refs/heads/master@{#292165}