This was an oversight from when I converted PendingResponse and various
other classes from being ref-counted to GC-allocated last minute - no
one takes care to keep all of them alive. Some are on the stack, and
some might be captured in another PendingResponse's JS::SafeFunction,
but ultimately, we need a better solution.
Since a PendingResponse is *always* the result of someone having created
a Request, let's just let that keep a list of each PendingResponse that
has been created for it, and visit them until they are resolved. After
that, they can be GC'd with no complaints.
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.
