message: Fix an warning about comparison between signed and unsigned integers
diff: @@ -39,9 +39,9 @@ P ParseProtoText(const std::string& txt) {
  std::string EraseSpace(const std::string& src) {
  std::string str = src;
  - int i = 0;
  + size_t i = 0;
  while (i < str.size()) {
  - int j = i;
  + size_t j = i;
  while (j < str.size() && str[j] == ' ') {
  ++j;
  }
  