These changes make sure that we postpone calls to
ReadableByteStreamControllerCommitPullIntoDescriptor until after all
pull-into descriptors have been filled up by
ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue.
Also pulls in the WPT test which was created in relation to this spec
change. The test verifies that a patched then() will see a null
byobRequest.
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
