Prototype additional WebNN operator implementations via DirectML

components/ml/mojom/web_platform_model.mojom

@@ -40,6 +40,8 @@ enum DevicePreference {
   // GPU. We do not directly return an error in this case to avoid directly
   // exposing user's GPU information.
   kGpu = 2,
+  // Prefers running the model inference on an NPU.
+  kNpu = 3,
 };
 
 [Stable, Extensible]