Demostrates that that `WeakRefs` with `MessagePort` already expose whether a cross-origin iframe has been destroyed. Try

1. click log
2. Choose one of
1. cross-origin
1. click sendPort1 cross-origin
2. click delete iframe
2. same-origin
1. click sendPort1 same-origin
2. click drop reference
3. force a garbage collection in devtools
4. click log

You'll see that the weak refs become null.