Eliminate busy count tracking in favor of just letting WorkerRefs, the existence of IPC actors, and (newly proposed) the presence of events in our main-thread ThrottledEventQueues/worker debugee queues keep a worker alive. This enables a simplification of the WorkerRunnable implementation. This is a spin-off follow-up from bug 1800659 which going to originally address this (but which quite reasonably was de-scoped).

One particular benefit is there is no need for runnables sent to the main thread to have a reference to the WorkerPrivate (to update the busy count), however it's likely that we would want to approximate the original design behavior by having the GC traversal logic in WorkerPrivate::Traverse that Worker::Traverse calls into:

• Check if there are any runnables in our mMainThreadDebuggeeEventTarget main thread-targeted ThrottledEventQueue. This is a new proposal with the filing of this bug, but an easy thing for us to check that could potentially allow us to continue to GC in more cases.
• We also would like to be aware of runnables resulting from Worker.postMessage; most practically having any runnables in the queue would be sufficient but would err on the side of GCing less.
• Check if there are StrongWorkerRefs. (IPCWorkerRefs would continue to not prevent GC, as would also be the case for WeakWorkerRefs.)

I have some local patches for this, so I take this bug.