Merge another ReplayStats into this one.

def merge(self, other):
    """Merge another ReplayStats into this one."""
    def merge_dict(a, b):
      for k, v in six.iteritems(b):
        a[k] += v

    self.replays += other.replays
    self.steps += other.steps
    self.camera_move += other.camera_move
    self.select_pt += other.select_pt
    self.select_rect += other.select_rect
    self.control_group += other.control_group
    merge_dict(self.maps, other.maps)
    merge_dict(self.races, other.races)
    merge_dict(self.unit_ids, other.unit_ids)
    merge_dict(self.valid_abilities, other.valid_abilities)
    merge_dict(self.made_abilities, other.made_abilities)
    merge_dict(self.valid_actions, other.valid_actions)
    merge_dict(self.made_actions, other.made_actions)
    self.crashing_replays |= other.crashing_replays
    self.invalid_replays |= other.invalid_replays