As WorkerGlobalScope's close() method should not be exposed to one of
its derived interfaces, ServiceWorkerGlobalScope, close() needs to be
moved to the derived interfaces which explicitly requires it. This
commit moves close() to DedicatedWorkerGlobalScope and
SharedWorkerGlobalScope.

Related issue: w3c/ServiceWorker#865

Extract close a worker algorithm that can be invoked by the close()
method of a worker. At the time of this patch, a dedicated worker and
a shared worker invoke it.