ServiceWorkers: Enable 'onmessage', not just addEventListener

Service Workers couldn't listen for posted messages via the old-school
'onmessage' hook - thanks to jyasskin for pointing this out.

Review URL: https://codereview.chromium.org/217203002

git-svn-id: svn://svn.chromium.org/blink/trunk@170430 bbb929c8-8fbe-4397-9dbb-9b2b20218538