Refactors event dispatching of NativeViewport into its own interface

mojo/services/public/interfaces/native_viewport/native_viewport.mojom

 // Copyright 2014 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.
 
 import "mojo/services/public/interfaces/geometry/geometry.mojom";
 import "mojo/services/public/interfaces/gpu/command_buffer.mojom";
 import "mojo/services/public/interfaces/input_events/input_events.mojom";
 import "mojo/services/public/interfaces/surfaces/surface_id.mojom";
 
 module mojo {
 
 [Client=NativeViewportClient]
 interface NativeViewport {
   // TODO(sky): having a create function is awkward. Should there be a factory
   // to create the NativeViewport that takes the size?
   Create(Size size) => (uint64 native_viewport_id);
   Show();
   Hide();
   Close();
   SetSize(Size size);
   SubmittedFrame(SurfaceId surface_id);
+  SetEventDispatcher(NativeViewportEventDispatcher dispatcher);
+};
+
+interface NativeViewportEventDispatcher {
+  OnEvent(Event event) => ();
 };
 
 interface NativeViewportClient {
   // OnSizeChanged() is sent at least once after the callback from Create() is
   // called.
   OnSizeChanged(Size size);
   OnDestroyed();
-  OnEvent(Event event) => ();
 };
 
 }