Update //third_party/protobuf to version 3.

third_party/protobuf/src/google/protobuf/compiler/plugin.proto

Index: third_party/protobuf/src/google/protobuf/compiler/plugin.proto
diff --git a/third_party/protobuf/src/google/protobuf/compiler/plugin.proto b/third_party/protobuf/src/google/protobuf/compiler/plugin.proto
index 77b888f37ededec94290a17a4f4f48db4a42101b..acaee1f49456c5554f6b8ce0f7a6ab38b8fda26e 100644
--- a/third_party/protobuf/src/google/protobuf/compiler/plugin.proto
+++ b/third_party/protobuf/src/google/protobuf/compiler/plugin.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
@@ -44,10 +44,13 @@
 // plugin should be named "protoc-gen-$NAME", and will then be used when the
 // flag "--${NAME}_out" is passed to protoc.
 
+syntax = "proto2";
 package google.protobuf.compiler;
 option java_package = "com.google.protobuf.compiler";
 option java_outer_classname = "PluginProtos";
 
+option go_package = "plugin_go";
+
 import "google/protobuf/descriptor.proto";
 
 // An encoded CodeGeneratorRequest is written to the plugin's stdin.