Update //third_party/protobuf to version 3.

third_party/protobuf/src/google/protobuf/unittest_empty.proto

Index: third_party/protobuf/src/google/protobuf/unittest_empty.proto
diff --git a/third_party/protobuf/src/google/protobuf/unittest_empty.proto b/third_party/protobuf/src/google/protobuf/unittest_empty.proto
index ab12d1fb9fff7c368168cb52859eae3954084410..36443e7e01259c186daeb45be6062397492a47ba 100644
--- a/third_party/protobuf/src/google/protobuf/unittest_empty.proto
+++ b/third_party/protobuf/src/google/protobuf/unittest_empty.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
@@ -35,3 +35,4 @@
 // This file intentionally left blank.  (At one point this wouldn't compile
 // correctly.)
 
+syntax = "proto2";