This builds on the existing ad-hoc ResourceLoader code for HTTP fetches
which works for files as well.
This also includes a test that checks that stylesheets loaded with the
"file" URL scheme actually work.
The main things missing is the CORS preflight cache and making
extract_header_list_values properly parse, validate and return split
values for the Access-Control headers.
This implements the following operations from section 4 of the Fetch
spec (https://fetch.spec.whatwg.org/#fetching):
- Fetch
- Main fetch
- Fetch response handover
- Scheme fetch
- HTTP fetch
- HTTP-redirect fetch
- HTTP-network-or-cache fetch (without caching)
It does *not* implement:
- HTTP-network fetch
- CORS-preflight fetch
Instead, we let ResourceLoader handle the actual networking for now,
which isn't ideal, but certainly enough to get enough functionality up
and running for most websites to not complain.
