Change AckEvent into request / response

mojo/examples/aura_demo/window_tree_host_mojo.cc

Index: mojo/examples/aura_demo/window_tree_host_mojo.cc
diff --git a/mojo/examples/aura_demo/window_tree_host_mojo.cc b/mojo/examples/aura_demo/window_tree_host_mojo.cc
index 4e94938473663fe89843fa541d5a5f78301301c2..8f8329f6fe5c51bb1f09826b6cdffb969d215e94 100644
--- a/mojo/examples/aura_demo/window_tree_host_mojo.cc
+++ b/mojo/examples/aura_demo/window_tree_host_mojo.cc
@@ -166,10 +166,8 @@ void WindowTreeHostMojo::OnDestroyed() {
   base::MessageLoop::current()->Quit();
 }
 
-void WindowTreeHostMojo::OnEvent(const Event& event) {
-  if (!event.location().is_null())
-    native_viewport_->AckEvent(event);
-
+void WindowTreeHostMojo::OnEvent(const Event& event,
+                                 const mojo::Callback<void()>& callback) {
   switch (event.action()) {
     case ui::ET_MOUSE_PRESSED:
     case ui::ET_MOUSE_DRAGGED:
@@ -194,6 +192,7 @@ void WindowTreeHostMojo::OnEvent(const Event& event) {
     }
     // TODO(beng): touch, etc.
   }
+  callback.Run();
 };
 
 }  // namespace examples