Move Mailbox from cc to gpu, and its traits to gpu/ipc

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 ac267db313f2ef43e5a685a0eba9a6bf80051c8e..6b2531f763a7041f02239136ce26b5bb3847c348 100644
--- a/gpu/ipc/gpu_command_buffer_traits.h
+++ b/gpu/ipc/gpu_command_buffer_traits.h
@@ -9,6 +9,10 @@
 #include "gpu/command_buffer/common/command_buffer.h"
 #include "gpu/gpu_export.h"
 
+namespace gpu {
+struct Mailbox;
+}
+
 namespace IPC {
 
 template <>
@@ -19,6 +23,14 @@ struct GPU_EXPORT ParamTraits<gpu::CommandBuffer::State> {
   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);
+};
+
 }  // namespace IPC
 
 #endif  // GPU_IPC_GPU_PARAM_TRAITS_H_