Improve clients.get(id)
This commit embraces the recently added environment concept in
enumerating service worker clients. Also, for an environment type
client, it references the environment's target browsing context's
active document's UI properties to set the visibilityState and the
focused state.
Related commit: https://github.com/w3c/ServiceWorker/commit/8b483b091e0f0bae6b698cf05d915c2029748ae0
