Implement ServiceWorkerGlobalObject registration
The current implementation is RELEASE_ASSERT_NOT_REACHED which means just evaluating `self.registration` inside of a ServiceWorker causes a crash. Easy to hit when inspecting a ServiceWorker.
That assertion has been dropped earlier today.
Services Workers are mostly stubs at the moment. It is not ready for testing.
So I see. This may be doable after bug 178882. I just filed this so I had a bugzilla bug to replace the FIXME, but I'll leave the code alone to avoid churn.