The lichess.org lobby stylesheet sometimes gets render-blocked,
resulting in a blank page.
The main fetch would queue work and then spin_until wait for a pending
preload result on the main thread. This re-entered the event loop and
stranded the queued PendingResponse callback behind a pile of nested
event processing. So the preload response existed, but the stylesheet
never made it through the normal handoff path.
I chose to follow the PreloadEntry.cpp model to address this and track
at most one pending preload PendingResponse for each FetchParams,
resolving it directly from the preload callback when the response
arrives. This removes the spin_until, keeps the handoff asynchronous,
and lets the blocked stylesheet finish loading normally.
If the current single-consumer preload path ever changes, this logic
must be widened to handle it.
If we've already fired off an error, calling stop_fetch() should
make no difference, other than stopping the Requests::Request.
Eventually, we'll probably want abort() and terminate() to
eventually stop the Requests::Request in an unobservable way.
This allows the Request to be cleaned up when it becomes inactive,
which in turn allows the GC to clean up the FetchController which is
indirectly captured by a root in the Request callbacks.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
Now that the heap has no knowledge about a JavaScript realm and is
purely for managing the memory of the heap, it does not make sense
to name this function to say that it is a non-realm variant.
