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

components/mus/ws/focus_controller.h

Index: components/mus/ws/focus_controller.h
diff --git a/components/mus/ws/focus_controller.h b/components/mus/ws/focus_controller.h
index d63f5655b144078600c206e05e5063f132503577..7181a4c2b95a5c51022d15afa1d25a29739de654 100644
--- a/components/mus/ws/focus_controller.h
+++ b/components/mus/ws/focus_controller.h
@@ -42,6 +42,10 @@ class FocusController : public ServerWindowDrawnTrackerObserver {
   bool CanBeFocused(ServerWindow* window) const;
   bool CanBeActivated(ServerWindow* window) const;
 
+  // Returns the closest activatable ancestor of |window|. Returns nullptr if
+  // there is no such ancestor.
+  ServerWindow* GetActivatableAncestorOf(ServerWindow* window) const;
+
   // Implementation of SetFocusedWindow().
   void SetFocusedWindowImpl(ServerWindow* window, ChangeSource change_source);
 
@@ -51,6 +55,11 @@ class FocusController : public ServerWindowDrawnTrackerObserver {
                            bool is_drawn) override;
 
   FocusControllerDelegate* delegate_;
+
+  ServerWindow* focused_window_;
+  ServerWindow* active_window_;
+
+  // Keeps track of the visibility of the focused and active window.
   scoped_ptr<ServerWindowDrawnTracker> drawn_tracker_;
 
   DISALLOW_COPY_AND_ASSIGN(FocusController);