NavigationPreloadManager.setHeaderValue should reject invalid HTTP header field values

[mfalken]

(Spinning off of the long thread at #920)
The WIP Blink implementation requires the string to be latin1-only and contain no \r, \n, or \0, rejecting with a TypeError if not.

This should be specced. Probably we'd point to the Fetch spec's definition, which is being worked on at whatwg/fetch#332.

[jakearchibald]

F2F: Yes.

[annevk]

This can be fixed now and tested for. I wonder if perhaps you should be even more restrictive and limit the range to 0x20-0x7E or some such.