mus: Remove the old client lib.

services/ui/public/cpp/window_tree_client_observer.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 SERVICES_UI_PUBLIC_CPP_WINDOW_TREE_CLIENT_OBSERVER_H_
-#define SERVICES_UI_PUBLIC_CPP_WINDOW_TREE_CLIENT_OBSERVER_H_
-
-namespace ui {
-
-class Window;
-class WindowTreeClient;
-
-class WindowTreeClientObserver {
- public:
-  virtual void OnWindowTreeFocusChanged(Window* gained_focus,
-                                        Window* lost_focus) {}
-
-  virtual void OnWindowTreeCaptureChanged(Window* gained_capture,
-                                          Window* lost_capture) {}
-
-  // Called early on in the destructor of WindowTreeClient; before any windows
-  // have been destroyed.
-  virtual void OnWillDestroyClient(WindowTreeClient* client) {}
-
-  // Called at the end of WindowTreeClient's destructor. At this point
-  // observers should drop all references to |client|.
-  virtual void OnDidDestroyClient(WindowTreeClient* client) {}
-
- protected:
-  virtual ~WindowTreeClientObserver() {}
-};
-
-}  // namespace ui
-
-#endif  // SERVICES_UI_PUBLIC_CPP_WINDOW_TREE_CLIENT_OBSERVER_H_