Correctly track cross-thread pending FetchEvents.
ServiceWorkerGlobalScopeProxy is an object that resides on the
main thread heap, but is passed to the embedder and called
on the (service) worker thread. As the object is GC managed
by the main thread, the worker thread cannot update the proxy
object with references to heap objects residing in its heap,
as the per-thread heap design assumes that each heap only keeps
local heap references (via Member<T> and similar.) References
across heaps must instead be kept and handled by explicit
This per-thread heap rule was not being followed for the tracking
of pending FetchEvents; adjust the map representation to do so.