Method AddCallback

test/callback_list_unittest.cc:58–64

        cb_reg_(cb_reg) {
  }
  void AddCallback() {
    if (!added_) {
      added_ = true;
      subscription_ =
          cb_reg_->Add(Bind(&Adder::IncrementTotal, Unretained(this)));
    }
  }
  void IncrementTotal() { total_++; }

  bool added() const { return added_; }