Pass site-for-cookies through Fetch

[jyasskin]

https://tools.ietf.org/html/draft-ietf-httpbis-cookie-same-site-00 and https://tools.ietf.org/html/draft-ietf-httpbis-rfc6265bis-02#section-5.2 define a "site for cookies" for clients which is passed through requests to determine which cookies to send. That's effectively a monkeypatch of this spec and HTML, and folks adding new kinds of clients or requests are likely to forget to look in the Cookies spec for variables they need to define, so it should move here and to HTML.

@mikewest and I think this is probably a Good First Issue for someone who wants to explore the Fetch and HTML specs.

[annevk]

(Removing good first issue as this would be a bit of work that's not as clear cut as what the people usually taking on "good first issue" issues expect in my experience.)