ServiceWorker

cf834f05 - Clarify target object of Client.postMessage can be null (#1274)

Clarify target object of Client.postMessage can be null (#1274) This change addresses the issue described in the second paragraph of https://github.com/w3c/ServiceWorker/issues/1042#issuecomment-270043959. This change gets |destination| variable initialized to null if the ServiceWorkerContainer object does not exist. Fixes #1042.