Method serialize

test/serialization/test_traits.cpp:66–70

       calculate_size_before_serialize_when_cache_is_needed) {
  struct S : public MinimalSerializable {
    void serialize(CodedOutputStream& os) const {
      calculate_serialized_size_before_serialize =
          calculate_serialized_size_called;
      MinimalSerializable::serialize(os);
    }
    size_t calculate_serialized_size() const {
      calculate_serialized_size_called = true;
      return MinimalSerializable::calculate_serialized_size();