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

Review URL: https://codereview.chromium.org/452883002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@179841 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs