Don't use ServiceWorkerContainer.unregister in layout tests

This patch removes all the calls to ServiceWorkerContainer.unregister()
from LayoutTests except for unregister.html. The next patch will remove
ServiceWorkerContainer.unregister method and the remaining tests.

* If worker is registered by the test itself, keep the registration
  object and call unregister().
* Otherwise, use the new helper function service_worker_unregister
  or directly call getRegistration and then registration.unregister().

BUG=404064

Review URL: https://codereview.chromium.org/580763002

git-svn-id: svn://svn.chromium.org/blink/trunk@182230 bbb929c8-8fbe-4397-9dbb-9b2b20218538