Distinguish the sender to the own thread and to the parent thread in DedicatedWorkerGlobalScope.

The WorkerGlobalScope stores a reference to the sender for the own thread, to
allow passing clones to sub-workers. The DedicatedWorkerGlobalScope
additionally keeps a reference to the sender for the (unique) parent thread,
to implement postMessage and memory management of the Worker object.
fixup! Store a pointer to the Worker in the DedicatedWorkerGlobalScope.
fixup! Implement DedicatedWorkerGlobalScope.postMessage.