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

cc/resources/transferable_resource.cc

 // Copyright 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.
 
 #include "base/logging.h"
 #include "cc/resources/returned_resource.h"
 #include "cc/resources/transferable_resource.h"
 
 namespace cc {
 
 TransferableResource::TransferableResource()
     : id(0),
-      sync_point(0),
       format(RGBA_8888),
-      target(0),
       filter(0),
       is_software(false) {}
 
 TransferableResource::~TransferableResource() {
 }
 
 ReturnedResource TransferableResource::ToReturnedResource() const {
   ReturnedResource returned;
   returned.id = id;
-  returned.sync_point = sync_point;
+  returned.sync_point = mailbox_holder.sync_point;
   returned.count = 1;
   return returned;
 }
 
 // static
 void TransferableResource::ReturnResources(
     const TransferableResourceArray& input,
     ReturnedResourceArray* output) {
   for (TransferableResourceArray::const_iterator it = input.begin();
        it != input.end(); ++it)
     output->push_back(it->ToReturnedResource());
 }
 
 }  // namespace cc