Fix crash caused by Canvas2DLayerBridge::mailboxReleased

Fix crash caused by Canvas2DLayerBridge::mailboxReleased

In case of the resource is lost but context is not lost, the mailboxInfo will
be removed from mailbox list, and so the reference to its parent Canvas2DLayerBridge
will also be cleared.

This fix is to return early to avoid crash by clearing the parent Canvas2DLayerBridge
twice.

BUG=401697
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179841

[Hongbo Min, 2014-08-08 11:24:11]

kbr@, junov@, please take a review. Thanks a lot!

[Justin Novosad, 2014-08-08 13:03:35]

On 2014/08/08 11:24:11, Hongbo Min wrote:
> kbr@, junov@, please take a review. Thanks a lot!

lgtm

[Justin Novosad, 2014-08-08 13:44:07]

The CQ bit was checked by junov@chromium.org

[commit-bot: I haz the power, 2014-08-08 13:45:09]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/hongbo.min@intel.com/452883002/1

[commit-bot: I haz the power, 2014-08-08 16:00:31]

Change committed as 179841

[Ken Russell (switch to Gerrit), 2014-08-08 17:44:10]

Is it possible to write a test for this? If so, please do.

[Hongbo Min, 2014-08-11 01:30:20]

On 2014/08/08 17:44:10, Ken Russell wrote:
> Is it possible to write a test for this? If so, please do.

Yes, i think so, it needs more test cases for Canvas2DLayerBridge.