Marks multiple urns (and their children) for deletion.

def MultiMarkForDeletion(self, urns):
    """Marks multiple urns (and their children) for deletion."""
    all_children_urns = self.RecursiveMultiListChildren(urns)

    urns.extend(collection.Flatten(itervalues(all_children_urns)))
    self._urns_for_deletion.update(urns)

    for obj in self.MultiOpen(urns):
      obj.OnDelete(deletion_pool=self)