[push-api] Push events should Soft Update service worker (#115)

Incoming push events should trigger the Service Worker's [Soft Update algorithm](https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#soft-update-algorithm) (currently only triggered by top-level page loads), to allow a Service Worker used only in the background to receive push events (and perhaps show notifications) to update itself.

The Soft Update should occur even if the event fails (event handler throws an exception, or rejects the `event.waitUntil` promise), as it's in such situations that it might be particularly urgent to update the Service Worker.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/115