If we run the remarking phase in the VM thread we'll crash if the reference processor is setup to use multi-threaded discovery.

We crash because as_Worker_thread() returns NULL in get_discovered_list():
  if (_discovery_is_mt) {
    // During a multi-threaded discovery phase,
    // each thread saves to its "own" list.
    Thread* thr = Thread::current();
    id = thr->as_Worker_thread()->id();
  } else {
    // single-threaded discovery, we save in round-robin
    // fashion to each of the lists.
    if (_processing_is_mt) {
      id = next_id();
    }
  }