athena: Fix switching windows with title-drag in split-view.

athena/wm/split_view_controller.h

Index: athena/wm/split_view_controller.h
diff --git a/athena/wm/split_view_controller.h b/athena/wm/split_view_controller.h
index 4e0d3ccb078f465d900f6f3f59fb62839622821f..8d8db77aca4bc9ebd03c76a2af3d44f5bc00d1b9 100644
--- a/athena/wm/split_view_controller.h
+++ b/athena/wm/split_view_controller.h
@@ -39,6 +39,9 @@ class ATHENA_EXPORT SplitViewController
   // making any necessary changes.
   void DeactivateSplitMode();
 
+  void ReplaceWindow(aura::Window* window,
+                     aura::Window* replace_with);
+
   aura::Window* left_window() { return left_window_; }
   aura::Window* right_window() { return right_window_; }