Update //third_party/protobuf to version 3.

third_party/protobuf/src/google/protobuf/stubs/once.h

Index: third_party/protobuf/src/google/protobuf/stubs/once.h
diff --git a/third_party/protobuf/src/google/protobuf/stubs/once.h b/third_party/protobuf/src/google/protobuf/stubs/once.h
index 9f1c5fc6469f5fa654a2d6b1c2eea9230ce5c1b8..9522b22a1c22657e5154734b86e4a417bb081d39 100644
--- a/third_party/protobuf/src/google/protobuf/stubs/once.h
+++ b/third_party/protobuf/src/google/protobuf/stubs/once.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
@@ -79,6 +79,7 @@
 #define GOOGLE_PROTOBUF_STUBS_ONCE_H__
 
 #include <google/protobuf/stubs/atomicops.h>
+#include <google/protobuf/stubs/callback.h>
 #include <google/protobuf/stubs/common.h>
 
 namespace google {
@@ -135,6 +136,24 @@ inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)(Arg*),
 
 #endif  // GOOGLE_PROTOBUF_NO_THREAD_SAFETY
 
+class GoogleOnceDynamic {
+ public:
+  GoogleOnceDynamic() : state_(GOOGLE_PROTOBUF_ONCE_INIT) { }
+
+  // If this->Init() has not been called before by any thread,
+  // execute (*func_with_arg)(arg) then return.
+  // Otherwise, wait until that prior invocation has finished
+  // executing its function, then return.
+  template<typename T>
+  void Init(void (*func_with_arg)(T*), T* arg) {
+    GoogleOnceInit<T>(&this->state_,
+                      func_with_arg,
+                      arg);
+  }
+ private:
+  ProtobufOnceType state_;
+};
+
 #define GOOGLE_PROTOBUF_DECLARE_ONCE(NAME) \
   ::google::protobuf::ProtobufOnceType NAME = GOOGLE_PROTOBUF_ONCE_INIT