Changes how window manager obtains WindowTree from mus

components/mus/ws/window_manager_factory_registry_observer.h

-// Copyright 2016 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 COMPONENTS_MUS_WS_WINDOW_MANAGER_FACTORY_REGISTRY_OBSERVER_H_
-#define COMPONENTS_MUS_WS_WINDOW_MANAGER_FACTORY_REGISTRY_OBSERVER_H_
-
-namespace mus {
-namespace ws {
-
-class WindowManagerFactoryService;
-
-class WindowManagerFactoryRegistryObserver {
- public:
-  // Called when the mojom::WindowManagerFactory associated with |service| has
-  // been obtained.
-  virtual void OnWindowManagerFactorySet(
-      WindowManagerFactoryService* service) = 0;
-
- protected:
-  virtual ~WindowManagerFactoryRegistryObserver() {}
-};
-
-}  // namespace ws
-}  // namespace mus
-
-#endif  // COMPONENTS_MUS_WS_WINDOW_MANAGER_FACTORY_REGISTRY_OBSERVER_H_