Specify the behavior of CookieStoreManager after ServiceWorkerRegistration has been unregistered

[szewai]

If a registration is unregistered, and there is no service worker using the registration, i.e. the registration will not be in use any more and its data might be cleared, should CookieStoreManager methods throw InvalidStateError? It seems it should, because we won't actually update the subscription list and cookie change event will not be fired.

[szewai]

cc @annevk @RupinMittal