If the client code reports the resource is lost, the mailbox should be
removed and also release the reference to DrawingBuffer, so that make
sure the DrawingBuffer has a chance to be destroyed.
    
BUG=390960

Review URL: https://codereview.chromium.org/415123002

git-svn-id: svn://svn.chromium.org/blink/trunk@179089 bbb929c8-8fbe-4397-9dbb-9b2b20218538