Promise<FrozenArray<T>> should become Promise<sequence<T>>

[domenic]

A few methods, namely:

• Clients's matchAll()
• Cache's matchAll() and keys()
• ServiceWorkerContainer's getRegistrations()

return Promise<FrozenArray<T>>.

As discussed in whatwg/webidl#1399, it doesn't really make sense to return frozen arrays as return values. The caller should just get back a sequence<T>.

It would be nice to clean this up so that the Web IDL spec can disallow using FrozenArray<T> in such scenarios.