Change AckEvent into request / response

mojo/services/native_viewport/native_viewport.mojom

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module mojo {
 
 struct Point {
   float x;
   float y;
 };
@@ skipping 27 matching lines @@
 };
 
 [Peer=NativeViewportClient]
 interface NativeViewport {
   Create(Rect bounds);
   Show();
   Hide();
   Close();
   SetBounds(Rect bounds);
   CreateGLES2Context(handle<message_pipe> gles2_client);
-  AckEvent(Event event);
 };
 
 [Peer=NativeViewport]
 interface NativeViewportClient {
   OnCreated();
   OnBoundsChanged(Rect bounds);
   OnDestroyed();
-  OnEvent(Event event);
+  OnEvent(Event event) => ();
 };
 
 }