Revert https://codereview.chromium.org/1291903002 (protobuf roll).

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 3aa31fa9ddc530414c6903c26791899a8d3febd4..3497f09fa6d41b7b38357e7f2610dfb5c087d3e4 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.
-// https://developers.google.com/protocol-buffers/
+// http://code.google.com/p/protobuf/
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
@@ -34,10 +34,8 @@
 //
 // 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.
@@ -64,17 +62,6 @@ 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 {