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

cc/resources/transferable_resource.cc

Index: cc/resources/transferable_resource.cc
diff --git a/cc/resources/transferable_resource.cc b/cc/resources/transferable_resource.cc
index 62f1bdb1641aa4896b1234742a9715380b121686..e7e9e24b3cab956ef56bdc69239ccda66a08442d 100644
--- a/cc/resources/transferable_resource.cc
+++ b/cc/resources/transferable_resource.cc
@@ -10,9 +10,7 @@ namespace cc {
 
 TransferableResource::TransferableResource()
     : id(0),
-      sync_point(0),
       format(RGBA_8888),
-      target(0),
       filter(0),
       is_software(false) {}
 
@@ -22,7 +20,7 @@ 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;
 }