Unfortunately during shutdown we can sometimes leak the runnable passed into
SystemGroup::Dispatch. It is leaked instead of being freed off main thread
because we sometimes are passing data which can only be freed on the main thread
safely to the main thread, and running the destructor on the wrong thread could
be really bad.
This is a really really gross workaround for that issue which helps to avoid the
XPCOM leak checker failures which were appearing on try.
MozReview-Commit-ID: GTfdxKnsTae
2321f62b21