[Athena] Hide the windows which used to be in split view in ActivateSplitMode()

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 c6c5e8e23c01f35624982a1a85fe7c0ca24bc7ef..fc476f0d4a9d6dafd27a4d9ab19a1e7df919aabd 100644
--- a/athena/wm/split_view_controller.h
+++ b/athena/wm/split_view_controller.h
@@ -105,6 +105,10 @@ class ATHENA_EXPORT SplitViewController
   // container_ coordinates (along the x axis).
   int separator_position_;
 
+  // Windows which should be hidden when the animation initiated by
+  // UpdateLayout() completes.
+  std::vector<aura::Window*> to_hide_;
+
   base::WeakPtrFactory<SplitViewController> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SplitViewController);