Gives user input focus to the current client and returns a Promise that resolves to the existing WindowClient.
The focused read-only property of the WindowClient interface is a Boolean that indicates whether the current client has focus.
The id attribute must return its associated service worker client's id.
See ¶4.2.8 navigate(url) of ServiceWorker whatwg spec.
the spec says this is of type any (?!)
The url attribute must return the context object's associated service worker client's serialized creation url.
The visibilityState read-only property of the WindowClient interface indicates the visibility of the current client.