Add gpu::MailboxHolder to hold state for a gpu::Mailbox

gpu/ipc/gpu_command_buffer_traits.h

Index: gpu/ipc/gpu_command_buffer_traits.h
diff --git a/gpu/ipc/gpu_command_buffer_traits.h b/gpu/ipc/gpu_command_buffer_traits.h
index 6b2531f763a7041f02239136ce26b5bb3847c348..ce854d2f27649349db399a6ea998626a48f3e97a 100644
--- a/gpu/ipc/gpu_command_buffer_traits.h
+++ b/gpu/ipc/gpu_command_buffer_traits.h
@@ -11,6 +11,7 @@
 
 namespace gpu {
 struct Mailbox;
+struct MailboxHolder;
 }
 
 namespace IPC {
@@ -31,6 +32,14 @@ struct GPU_EXPORT ParamTraits<gpu::Mailbox> {
   static void Log(const param_type& p, std::string* l);
 };
 
+template <>
+struct GPU_EXPORT ParamTraits<gpu::MailboxHolder> {
+  typedef gpu::MailboxHolder param_type;
+  static void Write(Message* m, const param_type& p);
+  static bool Read(const Message* m, PickleIterator* iter, param_type* p);
+  static void Log(const param_type& p, std::string* l);
+};
+
 }  // namespace IPC
 
 #endif  // GPU_IPC_GPU_PARAM_TRAITS_H_