Update //third_party/protobuf to version 3.

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..49d9adad0b699f7fe67c64f997b2761e8a73f8d1 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
@@ -32,36 +32,47 @@
 //  Based on original Protocol Buffers design by
 //  Sanjay Ghemawat, Jeff Dean, and others.
 //
-// This file contains messages for testing message_set_wire_format.
+// This file is similar to unittest_mset_wire_format.proto, but does not
+// have a TestMessageSet, so it can be downgraded to proto1.
+
+syntax = "proto2";
+
+import "google/protobuf/unittest_mset_wire_format.proto";
 
 package protobuf_unittest;
 
+option cc_enable_arenas = true;
 option optimize_for = SPEED;
 
-// A message with message_set_wire_format.
-message TestMessageSet {
-  option message_set_wire_format = true;
-  extensions 4 to max;
-}
-
 message TestMessageSetContainer {
-  optional TestMessageSet message_set = 1;
+  optional proto2_wireformat_unittest.TestMessageSet message_set = 1;
 }
 
 message TestMessageSetExtension1 {
-  extend TestMessageSet {
+  extend proto2_wireformat_unittest.TestMessageSet {
     optional TestMessageSetExtension1 message_set_extension = 1545008;
   }
   optional int32 i = 15;
 }
 
 message TestMessageSetExtension2 {
-  extend TestMessageSet {
+  extend proto2_wireformat_unittest.TestMessageSet {
     optional TestMessageSetExtension2 message_set_extension = 1547769;
   }
   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 {
@@ -69,4 +80,3 @@ message RawMessageSet {
     required bytes message = 3;
   }
 }
-