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

cc/resources/transferable_resource.h

Index: cc/resources/transferable_resource.h
diff --git a/cc/resources/transferable_resource.h b/cc/resources/transferable_resource.h
index 5c93d1c042f361b4aecd5c94cf21accfcf4c56b4..8ca9ffabe39f804c7a76f5771cfb34f5c39c58a1 100644
--- a/cc/resources/transferable_resource.h
+++ b/cc/resources/transferable_resource.h
@@ -10,7 +10,7 @@
 #include "base/basictypes.h"
 #include "cc/base/cc_export.h"
 #include "cc/resources/resource_format.h"
-#include "gpu/command_buffer/common/mailbox.h"
+#include "gpu/command_buffer/common/mailbox_holder.h"
 #include "ui/gfx/size.h"
 
 namespace cc {
@@ -29,12 +29,10 @@ struct CC_EXPORT TransferableResource {
                               ReturnedResourceArray* output);
 
   unsigned id;
-  unsigned sync_point;
   ResourceFormat format;
-  uint32 target;
   uint32 filter;
   gfx::Size size;
-  gpu::Mailbox mailbox;
+  gpu::MailboxHolder mailbox_holder;
   bool is_software;
 };