Define how WebSocket "fetching" works

[annevk]

A lot of the things specified in Fetch apply to WebSocket too, but since there's no easy way to update the protocol specification a lot of the details are scrambled in various drafts.

To clear this up we should specify all the monkey patching here, in a single place, and then point to HTML Standard here. Fetch then serves as its entry point to the WebSocket protocol and everything shared between Fetch and WebSocket should become a little clearer.

Bits elsewhere that need to be put here:

• HSTS
• https://w3c.github.io/webappsec-mixed-content/#websockets-integration
• https://w3c.github.io/webappsec-upgrade-insecure-requests/#websockets-integration
• https://w3c.github.io/webappsec-csp/#directive-connect-src

Perhaps even the entire bootstrapping WebSocket fetch could just be done in terms of Fetch. That would be ideal.

[annevk]

See also https://www.w3.org/Bugs/Public/show_bug.cgi?id=27869.

[annevk]

Reopen https://www.w3.org/Bugs/Public/show_bug.cgi?id=28841 if this does not work out.