Use virtual function to retrieve interface pointer

xfa/fwl/lightwidget/cfwl_checkbox.h

Index: xfa/fwl/lightwidget/cfwl_checkbox.h
diff --git a/xfa/fwl/lightwidget/cfwl_checkbox.h b/xfa/fwl/lightwidget/cfwl_checkbox.h
index eb23536e258235dc3656874fe69cc6b8aa5a3746..51e62943dc6bf41bdf5da133ca8fa0f2b0fca46a 100644
--- a/xfa/fwl/lightwidget/cfwl_checkbox.h
+++ b/xfa/fwl/lightwidget/cfwl_checkbox.h
@@ -15,6 +15,9 @@ class CFWL_CheckBox : public CFWL_Widget {
   CFWL_CheckBox();
   ~CFWL_CheckBox() override;
 
+  IFWL_CheckBox* GetWidget() override;
+  const IFWL_CheckBox* GetWidget() const override;
+
   static CFWL_CheckBox* Create();
 
   FWL_Error Initialize(const CFWL_WidgetProperties* pProperties = nullptr);