WebSocket: only set cookies if WebSocket handshake succeeded

[annevk]

See web-platform-tests/wpt#5213. This requires abstracting setting cookies somehow.

There's an additional question around cookies about at which point they are actually set and synchronized across "threads". A logical place for synchronization would be when Fetch queues a task that the response is in (all headers that is; though what about synchronous?) or when WebSocket establishes the connection and queues a task for that (it currently lets HTML deal with that, but maybe Fetch should take ownership).