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

components/mus/ws/focus_controller_observer.h

Index: components/mus/ws/focus_controller_observer.h
diff --git a/components/mus/ws/focus_controller_observer.h b/components/mus/ws/focus_controller_observer.h
index 015a51e7b99fa1da7bc2fee8408819d700d9f1ea..8edbae18c351eefcdc318e23f7a7d050f3bb36c6 100644
--- a/components/mus/ws/focus_controller_observer.h
+++ b/components/mus/ws/focus_controller_observer.h
@@ -2,8 +2,8 @@
 // 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_
+#ifndef COMPONENTS_MUS_WS_FOCUS_CONTROLLER_OBSERVER_H_
+#define COMPONENTS_MUS_WS_FOCUS_CONTROLLER_OBSERVER_H_
 
 namespace mus {
 namespace ws {
@@ -13,6 +13,8 @@ class ServerWindow;
 
 class FocusControllerObserver {
  public:
+  virtual void OnActivationChanged(ServerWindow* old_active_window,
+                                   ServerWindow* new_active_window) = 0;
   virtual void OnFocusChanged(FocusControllerChangeSource change_source,
                               ServerWindow* old_focused_window,
                               ServerWindow* new_focused_window) = 0;
@@ -24,4 +26,4 @@ class FocusControllerObserver {
 }  // namespace ws
 }  // namespace mus
 
-#endif  // COMPONENTS_MUS_WS_FOCUS_CONTROLLER_DELEGATE_H_
+#endif  // COMPONENTS_MUS_WS_FOCUS_CONTROLLER_OBSERVER_H_