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 0fa7bb4bc33dc5c18e0396cab64fafe5a7c5ea2a..5e4495d281ad61091c7e8c3cf5fb6209bdf9b505 100644
--- a/athena/wm/split_view_controller.h
+++ b/athena/wm/split_view_controller.h
@@ -35,6 +35,12 @@ class SplitViewController : public BezelController::ScrollDelegate,
 
   void ActivateSplitMode(aura::Window* left, aura::Window* right);
 
+  void ReplaceWindow(aura::Window* window,
+                     aura::Window* replace_with);
+
+  aura::Window* left_window() { return left_window_; }
+  aura::Window* right_window() { return right_window_; }
+
  private:
   enum State {
     // Split View mode is not active. |left_window_| and |right_window| are