SW sample code in explainer doesn't deal with worker being killed

[mkruisselbrink]

In the "edit a file" example you add event listeners while handling another event. If the service worker gets killed it will never be able to deliver that update event to the service worker since the worker doesn't re-register for th event.

More generally in all the service worker code you do asynchronous operations in event listeners, but don't call "waitUntil" on the event, so the browser will have no way of knowing that it should keep the service worker alive.