ml: migrate mojom to OnceCallback

Callback will be deprecated in generated mojom bindings.
Move to OnceCallback now so it can be compatible with newer libchrome.

BUG=chromium:1054279
TEST=emerge

Change-Id: I00b9972fdd4a32ab704242f385ea561327fa7571
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2154300
Reviewed-by: Andrew Moylan <amoylan@chromium.org>
Tested-by: Qijiang Fan <fqj@google.com>
Commit-Queue: Qijiang Fan <fqj@google.com>
diff --git a/ml/machine_learning_service_impl.h b/ml/machine_learning_service_impl.h
index 5ebe3e3..b3d2cb5 100644
--- a/ml/machine_learning_service_impl.h
+++ b/ml/machine_learning_service_impl.h
@@ -39,13 +39,13 @@
   void LoadBuiltinModel(
       chromeos::machine_learning::mojom::BuiltinModelSpecPtr spec,
       chromeos::machine_learning::mojom::ModelRequest request,
-      const LoadBuiltinModelCallback& callback) override;
+      LoadBuiltinModelCallback callback) override;
 
   // chromeos::machine_learning::mojom::MachineLearningService:
   void LoadFlatBufferModel(
       chromeos::machine_learning::mojom::FlatBufferModelSpecPtr spec,
       chromeos::machine_learning::mojom::ModelRequest request,
-      const LoadFlatBufferModelCallback& callback) override;
+      LoadFlatBufferModelCallback callback) override;
 
   // Metadata required to load builtin models. Initialized at construction.
   const std::map<chromeos::machine_learning::mojom::BuiltinModelId,