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

components/mus/ws/focus_controller_delegate.h

Index: components/mus/ws/focus_controller_delegate.h
diff --git a/components/mus/ws/focus_controller_delegate.h b/components/mus/ws/focus_controller_delegate.h
index 11ef9ad7e499c00795fe63ac5703a405d3ad9248..783ace7d7994ad6d9dbd559b0617029f4c5fc6d5 100644
--- a/components/mus/ws/focus_controller_delegate.h
+++ b/components/mus/ws/focus_controller_delegate.h
@@ -13,6 +13,9 @@ class ServerWindow;
 
 class FocusControllerDelegate {
  public:
+  virtual bool CanHaveActiveChildren(ServerWindow* window) const = 0;
+  virtual void OnActivationChanged(ServerWindow* old_active_window,
+                                   ServerWindow* new_active_window) = 0;
   virtual void OnFocusChanged(ServerWindow* old_focused_window,
                               ServerWindow* new_focused_window) = 0;