Pull new version of protobuf sources.

third_party/protobuf/src/google/protobuf/unittest_mset.proto

Index: third_party/protobuf/src/google/protobuf/unittest_mset.proto
diff --git a/third_party/protobuf/src/google/protobuf/unittest_mset.proto b/third_party/protobuf/src/google/protobuf/unittest_mset.proto
index 3497f09fa6d41b7b38357e7f2610dfb5c087d3e4..3aa31fa9ddc530414c6903c26791899a8d3febd4 100644
--- a/third_party/protobuf/src/google/protobuf/unittest_mset.proto
+++ b/third_party/protobuf/src/google/protobuf/unittest_mset.proto
@@ -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
@@ -34,8 +34,10 @@
 //
 // This file contains messages for testing message_set_wire_format.
 
+syntax = "proto2";
 package protobuf_unittest;
 
+option cc_enable_arenas = true;
 option optimize_for = SPEED;
 
 // A message with message_set_wire_format.
@@ -62,6 +64,17 @@ message TestMessageSetExtension2 {
   optional string str = 25;
 }
 
+// This message was used to generate
+// //net/proto2/python/internal/testdata/message_set_message, but is commented
+// out since it must not actually exist in code, to simulate an "unknown"
+// extension.
+// message TestMessageSetUnknownExtension {
+//   extend TestMessageSet {
+//     optional TestMessageSetUnknownExtension message_set_extension = 56141421;
+//   }
+//   optional int64 a = 1;
+// }
+
 // MessageSet wire format is equivalent to this.
 message RawMessageSet {
   repeated group Item = 1 {