Fix ownership bug with ViewObserver::OnViewInputEvent

mojo/services/public/cpp/view_manager/view_observer.h

 // 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.
 
 #ifndef MOJO_SERVICES_PUBLIC_CPP_VIEW_MANAGER_VIEW_OBSERVER_H_
 #define MOJO_SERVICES_PUBLIC_CPP_VIEW_MANAGER_VIEW_OBSERVER_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
 
 #include "mojo/services/public/interfaces/input_events/input_events.mojom.h"
 
 namespace mojo {
 namespace view_manager {
 
 class View;
 
 class ViewObserver {
  public:
   enum DispositionChangePhase {
     DISPOSITION_CHANGING,
     DISPOSITION_CHANGED
   };
 
   virtual void OnViewDestroy(View* view, DispositionChangePhase phase) {}
 
-  virtual void OnViewInputEvent(View* view, EventPtr event) {}
+  virtual void OnViewInputEvent(View* view, const EventPtr& event) {}
 
  protected:
   virtual ~ViewObserver() {}
 };
 
 }  // namespace view_manager
 }  // namespace mojo
 
 #endif  // MOJO_SERVICES_PUBLIC_CPP_VIEW_MANAGER_VIEW_OBSERVER_H_