Oilpan: have SharedWorker trace its message port.

Also remove the unused AbstractWorker::asID() helper.

R=haraken@chromium.org,ager@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/371403002

git-svn-id: svn://svn.chromium.org/blink/trunk@177673 bbb929c8-8fbe-4397-9dbb-9b2b20218538