Update request's destination field to match Fetch

[annevk]

In whatwg/fetch#211 @igrigorik changed the design of request's destination field. We should update HTML to use the new values. Can I convince you to do that Ilya?

[igrigorik]

Yes, I can take a run at it (albeit in a few weeks, I'll be offline for next while).

That said.. @domenic looks like you're already making good progress. Let me know if/what you're working on, so we don't step on each other's toes.

[domenic]

Not planning on working on this anymore at this point, if you're willing to do it. I just updated script-related stuff since I had to change that area anyway for service worker.

[igrigorik]

OK, I'll take a pass over the rest.

[annevk]

Fixed by #1470 and 5e8f96a.