message: [gRPC] Uncomment MessageBuilder
  The line where the MessageBuilder was
  constructed was commented out (perhaps
  an oversight).
diff: @@ -12,7 +12,8 @@ class GreeterServiceImpl final : public Greeter::Service {
  grpc::ServerContext *context,
  const flatbuffers::grpc::Message<HelloRequest> *request_msg,
  flatbuffers::grpc::Message<HelloReply> *response_msg) override {
  - // flatbuffers::grpc::MessageBuilder mb_;
  + flatbuffers::grpc::MessageBuilder mb_;
  +
  // We call GetRoot to "parse" the message. Verification is already
  // performed by default. See the notes below for more details.
  const HelloRequest *request = request_msg->GetRoot();
  