updatefound event in service worker context seems unreliable

[wanderview]

Currently we expose the service workers current registration as self.registration. In theory, the service worker can add an updatefound event handle on the registration to know when an event happens. This event in a service context, though, seems unreliable since it doesn't use ExtendableEvent. For example:

1. The updatefound event does not start the service worker if its been stopped. (AFAICT)
2. The updatefound event does not provide an event.waitUntil() method.

Should updatefound be usable in a service worker context? If so, should the browser start the worker to fire this event and should it be an ExtendableEvent? If its an ExtendableEvent, should waitUntil() delay the installation of the update?