Allow EmbedRoot to be called multiple times.

mojo/services/public/cpp/view_manager/view_event_dispatcher.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_EVENT_DISPATCHER_H_
-#define MOJO_SERVICES_PUBLIC_CPP_VIEW_MANAGER_VIEW_EVENT_DISPATCHER_H_
-
-#include "mojo/services/public/interfaces/input_events/input_events.mojom.h"
-
-namespace mojo {
-namespace view_manager {
-
-class View;
-
-// A ViewEventDispatcher is provided by the application rendering at the root
-// of a Node hierarchy. It is responsible for targeting input events to the
-// relevant Views. This allows window manager features like focus, activation,
-// modality, etc. to be implemented.
-class ViewEventDispatcher {
- public:
-  virtual void DispatchEvent(View* target, EventPtr event) = 0;
-
- protected:
-  virtual ~ViewEventDispatcher() {}
-};
-
-}  // namespace view_manager
-}  // namespace mojo
-
-#endif  // MOJO_SERVICES_PUBLIC_CPP_VIEW_MANAGER_VIEW_EVENT_DISPATCHER_H_