Converts focusmanager from delegate to observer

components/mus/ws/focus_controller_delegate.h

-// Copyright 2015 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_FOCUS_CONTROLLER_DELEGATE_H_
-#define COMPONENTS_MUS_WS_FOCUS_CONTROLLER_DELEGATE_H_
-
-namespace mus {
-
-namespace ws {
-
-class ServerWindow;
-
-class FocusControllerDelegate {
- public:
-  virtual void OnFocusChanged(ServerWindow* old_focused_window,
-                              ServerWindow* new_focused_window) = 0;
-
- protected:
-  ~FocusControllerDelegate() {}
-};
-
-}  // namespace ws
-
-}  // namespace mus
-
-#endif  // COMPONENTS_MUS_WS_FOCUS_CONTROLLER_DELEGATE_H_