Fix crash when DecrementCapturerCount() makes a delayed WasHidden() call during WebContentsImpl dest

Fix crash when DecrementCapturerCount() makes a delayed WasHidden() call during WebContentsImpl destruction.

Two parts:

1. WebContentsImpl::DecrementCapturerCount() will restore the true "WasHidden" status of the RenderWidgetHost once the counter reaches zero.  However, DecrementCapturerCount() can be called [indirectly] from the WebContentsImpl destructor.  In this case, it should not attempt to change/notify anything.

2. It's possible for ThumbnailTabHelper::WidgetHidden() to call ThumbnailTabHelper::UpdateThumbnailIfNecessary() with a NULL pointer.  Added a NULL check.

BUG=175275, 130097
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=181876

[miu, 2013-02-11 23:47:07]

Nick: PTAL.

Ben: Need OWNERS approval for both files.

Thanks,
Yuri

[miu, 2013-02-11 23:56:33]

s/ben/sky/ in reviewers list

sky: Given you've looked at my original change recently, I think you'll be more
appropriate for OWNERS approval here.  PTAL at both files.

Thanks,
Yuri

[ncarter (slow), 2013-02-12 00:03:00]

lgtm

LGTM

[sky, 2013-02-12 00:39:02]

LGTM

[commit-bot: I haz the power, 2013-02-12 00:59:53]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/miu@chromium.org/12209104/4002

[commit-bot: I haz the power, 2013-02-12 06:15:28]

Change committed as 181876