Re: [ServiceWorker] ServiceWorkerClient to Client (#588)

If `getAll` returns different types, given that workers are still pretty rare, I'm worried that code would appear to work, go live, then start failing because a worker was present.

```
interface BasicClient {
  readonly attribute USVString url;
  readonly attribute ServiceWorkerRegistration? serviceWorkerRegistration;
  void postMessage(any message, optional sequence<Transferable> transfer);
}

interface WindowClient : BasicClient {
  readonly attribute VisibilityState visibilityState;
  readonly attribute boolean focused;
  readonly attribute ContextFrameType frameType;
  Promise<focus> focus();
}
```

Assuming you mean the above, it isn't too bad right now. If your code encounters a `BasicClient` for the first time, `.visibilityState != "visible"`, `.focused != true`, `frameType != "top-level". If you call `.focus()` you're going to throw an error, that won't often be different from rejecting, but I don't know how much money I'd bet on that.

Adding stuff to `WindowClient` in future may be tough.

Assuming we went with the above, what API would you prefer for opening a window, considering a constructor here is undesirable? `clients.openWindow(url)`, `WindowClient.open(url)`?

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