Fix for crash in Canvas2DLayerBridge::releasedMailboxInfo()

BUG=343944
R=senorblanco@chromium.org

Review URL: https://codereview.chromium.org/166433006

git-svn-id: svn://svn.chromium.org/blink/trunk@167224 bbb929c8-8fbe-4397-9dbb-9b2b20218538