Change IFWL_Widget to store a single delegate.

xfa/fwl/core/cfwl_widget.h

Index: xfa/fwl/core/cfwl_widget.h
diff --git a/xfa/fwl/core/cfwl_widget.h b/xfa/fwl/core/cfwl_widget.h
index 28a76661ef3b498f77b18a92010c21a601bc17f1..7938e241f5c91e7a9c7fb815a9054b5a13d7b720 100644
--- a/xfa/fwl/core/cfwl_widget.h
+++ b/xfa/fwl/core/cfwl_widget.h
@@ -51,8 +51,8 @@ class CFWL_Widget {
   FWL_Error DrawWidget(CFX_Graphics* pGraphics,
                        const CFX_Matrix* pMatrix = nullptr);
 
-  IFWL_WidgetDelegate* GetCurrentDelegate();
-  void SetCurrentDelegate(IFWL_WidgetDelegate*);
+  std::unique_ptr<IFWL_WidgetDelegate> ReleaseDelegate();
+  void SetDelegate(std::unique_ptr<IFWL_WidgetDelegate> pDelegate);
 
  protected:
   void Initialize();