Use virtual function to retrieve interface pointer

xfa/fwl/lightwidget/cfwl_datetimepicker.h

Index: xfa/fwl/lightwidget/cfwl_datetimepicker.h
diff --git a/xfa/fwl/lightwidget/cfwl_datetimepicker.h b/xfa/fwl/lightwidget/cfwl_datetimepicker.h
index 4348b6cd38aefad90aab8c3ac19c2b60a853a77d..d727729e9d0d988a52b30da56ca6de3d195c6f52 100644
--- a/xfa/fwl/lightwidget/cfwl_datetimepicker.h
+++ b/xfa/fwl/lightwidget/cfwl_datetimepicker.h
@@ -14,6 +14,9 @@ class CFWL_DateTimePicker : public CFWL_Widget {
  public:
   static CFWL_DateTimePicker* Create();
 
+  IFWL_DateTimePicker* GetWidget() override;
+  const IFWL_DateTimePicker* GetWidget() const override;
+
   FWL_Error Initialize(const CFWL_WidgetProperties* pProperties = nullptr);
   FWL_Error SetToday(int32_t iYear, int32_t iMonth, int32_t iDay);
   FWL_Error GetEditText(CFX_WideString& wsText);