By creating a new FetchController here, we prevent the original
FetchController from affecting the actual ongoing request. This is
necessary to allow FetchController::stop_fetch() to stop an ongoing
fetch in the subsequent commit.
The purpose of the copy appears to be only to change the
httpFetchParams' request over to the newly-cloned httpRequest, so all
existing references should refer back to the originals rather than new
instances.
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.
