Support returning ServiceWorker Client environments from `Clients.matchAll()`

[wanderview]

Now that we support matching uncontrolled Client environments it seems we should reasonably be able to get a Client object for a ServiceWorker. I believe the spec mostly already supports this as it doesn't explicitly exclude ServiceWorker environments. We just need to add a "serviceworker" enum value to ClientType.

[jakearchibald]

I like the idea of eventually making the clients API available to documents, so yeah, it makes sense to surface service worker clients here.

[wanderview]

Well, this is possible even without window.clients. If you have two service workers registered for the same origin and one does clients.matchAll({ includeUncontrolled: true}) it should in theory include the other service worker as a Client if it happens to be running.

[jungkees]

I'll review #1074.

I like the idea of eventually making the clients API available to documents,

That seems like a good direction, but I'm worried the API as-is seems a bit awkward for the case between non-service worker clients. window.clients.get(idOfAnotherWindow).then(client => client.postMessage(msg)); will land on navigator.serviceWorker. This seems to be a separate issue if we would work on that.

[wanderview]

Yes, that is issue #955.

That seems like a good direction, but I'm worried the API as-is seems a bit awkward for the case between non-service worker clients. window.clients.get(idOfAnotherWindow).then(client => client.postMessage(msg)); will land on navigator.serviceWorker. This seems to be a separate issue if we would work on that.

My proposal to fix that is in this comment:

#955 (comment)