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.
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