Re: [ServiceWorker] Foreign fetch (#751)

I updated the explainer with what I think were the resolutions from the F2F. Also updated the spec to get rid of the withCredentials flag, and instead have a list of origins when registering for foreign fetch.
I didn't specify any kind of `makeVisible` method yet, but did change it to now turn all responses in opaque responses.

---
Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/pull/751#issuecomment-176495914