Re: [ServiceWorker] Clients & postMessage (#609)

Additionally, I think we should make `postMessage` return a promise which rejects on unsuccessful delivery. After a push message, a SW may find a focused & visible client and post a message to it, but that fails because the tab as since been closed. Developers should be able to catch that case and show a notification instead.

This also allows us to reject posts to "asleep" tabs. https://github.com/slightlyoff/ServiceWorker/issues/626

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