[whatwg/fetch] CORS preflight request and window (#1242)

https://fetch.spec.whatwg.org/#cors-preflight-fetch defines _preflight_ as follows.

> Let _preflight_ be a new request whose method is `OPTIONS`, URL is _request_’s current URL, initiator is _request_’s initiator, destination is _request_’s destination, origin is _request_’s origin, referrer is _request_’s referrer, referrer policy is _request_’s referrer policy, mode is "cors", tainted origin flag is _request_’s tainted origin flag, and response tainting is "cors".
 
Note that _preflight_'s [window](https://fetch.spec.whatwg.org/#ref-for-concept-request-window) is not set, which means its value is "client". For usual requests the value is replaced with request's client or "window" in https://fetch.spec.whatwg.org/#concept-fetch but https://fetch.spec.whatwg.org/#cors-preflight-fetch calls https://fetch.spec.whatwg.org/#concept-http-network-or-cache-fetch directly and _preflight_'s window remains "client", which is somewhat an unexpected input there.

I believe 407 should result in preflight failure, so we should set _preflight_'s [window](https://fetch.spec.whatwg.org/#ref-for-concept-request-window) to "no-window".

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