Let canvas 2D decide how to handle the case of resource lost reported by client

We must clear and release some internal references even if the mailbox resource
is treated as lost from client code, so that no memory leaking happens.


BUG=390960

Review URL: https://codereview.chromium.org/369043003

git-svn-id: svn://svn.chromium.org/blink/trunk@178736 bbb929c8-8fbe-4397-9dbb-9b2b20218538