Speculative fix to the crash when calling Canvas2DLayerBridge::mailboxReleased

The reference to Canvas2DLayerBridge held by MailboxInfo must be transferred
when it is copied out, otherwise it would canuse undefined behaviour.

BUG=405289

Review URL: https://codereview.chromium.org/521103002

git-svn-id: svn://svn.chromium.org/blink/trunk@181273 bbb929c8-8fbe-4397-9dbb-9b2b20218538