After the changes in the previous part, actors are now removed from their
manager's managed list before invoking ActorDestroy. There were a few places in
tree which specifically checked for the old behaviour, due to calls from the
managed actor during ActorDestroy to potentially trigger clean-up.
This patch adjusts these checks to instead expect a count of `0` as the actor
will now already have been removed at that point.
Differential Revision: https://phabricator.services.mozilla.com/D198841