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

cc/test/fake_delegated_renderer_layer_impl.cc

Index: cc/test/fake_delegated_renderer_layer_impl.cc
diff --git a/cc/test/fake_delegated_renderer_layer_impl.cc b/cc/test/fake_delegated_renderer_layer_impl.cc
index 4057736a31250a8a99555d3dea055ae380d8fca7..8f4c1048c86334516f55db6a6e82f08f559c9def 100644
--- a/cc/test/fake_delegated_renderer_layer_impl.cc
+++ b/cc/test/fake_delegated_renderer_layer_impl.cc
@@ -30,7 +30,8 @@ static ResourceProvider::ResourceId AddResourceToFrame(
     ResourceProvider::ResourceId resource_id) {
   TransferableResource resource;
   resource.id = resource_id;
-  resource.target = resource_provider->TargetForTesting(resource_id);
+  resource.mailbox_holder.texture_target =
+      resource_provider->TargetForTesting(resource_id);
   frame->resource_list.push_back(resource);
   return resource_id;
 }