Refactors event dispatching of NativeViewport into its own interface

examples/surfaces_app/surfaces_app.cc

Index: examples/surfaces_app/surfaces_app.cc
diff --git a/examples/surfaces_app/surfaces_app.cc b/examples/surfaces_app/surfaces_app.cc
index 1dd8bc2d6cde71dd2f9c31256fa58ee43d8f5c49..4331654f4c9218afa679c92ab055de196bcaf920 100644
--- a/examples/surfaces_app/surfaces_app.cc
+++ b/examples/surfaces_app/surfaces_app.cc
@@ -105,10 +105,6 @@ class SurfacesApp : public ApplicationDelegate,
   // NativeViewportClient implementation.
   virtual void OnSizeChanged(mojo::SizePtr size) override {}
   virtual void OnDestroyed() override {}
-  virtual void OnEvent(mojo::EventPtr event,
-                       const mojo::Callback<void()>& callback) override {
-    callback.Run();
-  }
 
  private:
   void OnCreatedNativeViewport(uint64_t native_viewport_id) {}