Make cold_threshold_ look used to fix warning

This field isn't used yet, but it will be in the future so we don't want
to remove it. This fixes the warning in the meantime.

src/google/protobuf/compiler/cpp/cpp_message.cc

@@ -521,7 +521,11 @@ class ColdChunkSkipper {
 // Tuning parameters for ColdChunkSkipper.
 const double kColdRatio = 0.005;
 
-bool ColdChunkSkipper::IsColdChunk(int chunk) { return false; }
+bool ColdChunkSkipper::IsColdChunk(int chunk) {
+  // Mark this variable as used until it is actually used
+  (void) cold_threshold_;
+  return false;
+}
 
 void ColdChunkSkipper::OnStartChunk(int chunk, int cached_has_bit_index,
                                     const std::string& from,