mus: Allow the WM to specify the windows that can have active children.

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 {
 
-enum class FocusControllerChangeSource;
 class ServerWindow;
 
-class FocusControllerObserver {
+class FocusControllerDelegate {
  public:
-  virtual void OnFocusChanged(FocusControllerChangeSource change_source,
-                              ServerWindow* old_focused_window,
-                              ServerWindow* new_focused_window) = 0;
+  virtual bool CanHaveActiveChildren(ServerWindow* window) const = 0;
 
  protected:
-  ~FocusControllerObserver() {}
+  ~FocusControllerDelegate() {}
 };
 
 }  // namespace ws
+
 }  // namespace mus
 
 #endif  // COMPONENTS_MUS_WS_FOCUS_CONTROLLER_DELEGATE_H_