Migrate some tests to use ServiceWorkerRegistration.unregister()

ServiceWorkerContainer.unregister() is going away but the change will be
done in steps. This patch updates some select tests focused on
unregistration to use the new unregister() function.

BUG=404064

Review URL: https://codereview.chromium.org/474193002

git-svn-id: svn://svn.chromium.org/blink/trunk@180354 bbb929c8-8fbe-4397-9dbb-9b2b20218538