Update //third_party/protobuf to version 3.

third_party/protobuf/src/google/protobuf/compiler/subprocess.h

Index: third_party/protobuf/src/google/protobuf/compiler/subprocess.h
diff --git a/third_party/protobuf/src/google/protobuf/compiler/subprocess.h b/third_party/protobuf/src/google/protobuf/compiler/subprocess.h
index de9fce9ec9385c7f6ab15b5b6672e9610a20da0b..2513863144be7c534068158fae2fec3f0a439924 100644
--- a/third_party/protobuf/src/google/protobuf/compiler/subprocess.h
+++ b/third_party/protobuf/src/google/protobuf/compiler/subprocess.h
@@ -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
@@ -53,7 +53,7 @@ class Message;
 namespace compiler {
 
 // Utility class for launching sub-processes.
-class Subprocess {
+class LIBPROTOC_EXPORT Subprocess {
  public:
   Subprocess();
   ~Subprocess();