Update //third_party/protobuf to version 3.

third_party/protobuf/src/google/protobuf/reflection_ops.cc

Index: third_party/protobuf/src/google/protobuf/reflection_ops.cc
diff --git a/third_party/protobuf/src/google/protobuf/reflection_ops.cc b/third_party/protobuf/src/google/protobuf/reflection_ops.cc
index f00997c69f0fcac9d6565144ccee0d787cc2341d..4629dec2510b1cf80dc57f99c39a653193e50a17 100644
--- a/third_party/protobuf/src/google/protobuf/reflection_ops.cc
+++ b/third_party/protobuf/src/google/protobuf/reflection_ops.cc
@@ -1,6 +1,6 @@
 // Protocol Buffers - Google's data interchange format
 // Copyright 2008 Google Inc.  All rights reserved.
-// http://code.google.com/p/protobuf/
+// https://developers.google.com/protocol-buffers/
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
@@ -56,7 +56,9 @@ void ReflectionOps::Merge(const Message& from, Message* to) {
 
   const Descriptor* descriptor = from.GetDescriptor();
   GOOGLE_CHECK_EQ(to->GetDescriptor(), descriptor)
-    << "Tried to merge messages of different types.";
+    << "Tried to merge messages of different types "
+    << "(merge " << descriptor->full_name()
+    << " to " << to->GetDescriptor()->full_name() << ")";
 
   const Reflection* from_reflection = from.GetReflection();
   const Reflection* to_reflection = to->GetReflection();