Cookies: Integration with external specs.

While we have the cookies spec open, I think we should take a closer look
at how that specification interacts with others. In particular, two things
come to mind:

* We should formalize the integration with Fetch (see step 11.1 of
https://fetch.spec.whatwg.org/#http-network-or-cache-fetch and 9.3 of
https://fetch.spec.whatwg.org/#http-network-fetch).

* https://w3c.github.io/webappsec-csp/cookies/ defines a scoping mechanism
for `document.cookies` and `Set-Cookie` via a monkey-patch to the RFC.
Putting some sort of generic policy hook into the document (either via
explicit dependencies, as in that document, or via some sort of registry of
delegates) seems valuable.

What do you folks think?

-mike