message: Fix GYP Linux build.
  Apparently, clang++ on Linux GYP build warns of copying in range-based for
  loop. To suppress it, this CL changes the binding to reference.
diff: @@ -197,7 +197,7 @@ bool CharacterGroupFinder::FindMatch(
  const std::vector<char32_t> &target) const {
  for (const char32_t codepoint : target) {
  // Single codepoint check
  - for (const auto [left, right] : single_codepoint_ranges_) {
  + for (const auto &[left, right] : single_codepoint_ranges_) {
  if (left <= codepoint && codepoint <= right) {
  return true;
  }
  