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

gpu/ipc/gpu_command_buffer_traits.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef GPU_IPC_GPU_PARAM_TRAITS_H_
 #define GPU_IPC_GPU_PARAM_TRAITS_H_
 
 #include "ipc/ipc_message_utils.h"
 #include "gpu/command_buffer/common/command_buffer.h"
 #include "gpu/gpu_export.h"
 
 namespace gpu {
 struct Mailbox;
+struct MailboxHolder;
 }
 
 namespace IPC {
 
 template <>
 struct GPU_EXPORT ParamTraits<gpu::CommandBuffer::State> {
   typedef gpu::CommandBuffer::State 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);
 };
 
 template<>
 struct GPU_EXPORT ParamTraits<gpu::Mailbox> {
   typedef gpu::Mailbox 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);
 };
 
+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_