Use virtual function to retrieve interface pointer

xfa/fwl/lightwidget/cfwl_pushbutton.cpp

Index: xfa/fwl/lightwidget/cfwl_pushbutton.cpp
diff --git a/xfa/fwl/lightwidget/cfwl_pushbutton.cpp b/xfa/fwl/lightwidget/cfwl_pushbutton.cpp
index 7c8a9fa64dbcdc3b8c1c5aa4bfbf90ec13528474..b1031692ddd058b3c6ec01e38e6696a314b63ccb 100644
--- a/xfa/fwl/lightwidget/cfwl_pushbutton.cpp
+++ b/xfa/fwl/lightwidget/cfwl_pushbutton.cpp
@@ -8,6 +8,14 @@
 
 #include <memory>
 
+IFWL_PushButton* CFWL_PushButton::GetWidget() {
+  return static_cast<IFWL_PushButton*>(m_pIface.get());
+}
+
+const IFWL_PushButton* CFWL_PushButton::GetWidget() const {
+  return static_cast<IFWL_PushButton*>(m_pIface.get());
+}
+
 CFWL_PushButton* CFWL_PushButton::Create() {
   return new CFWL_PushButton;
 }
@@ -25,7 +33,7 @@ FWL_Error CFWL_PushButton::Initialize(
   if (ret != FWL_Error::Succeeded) {
     return ret;
   }
-  m_pIface = pPushButton.release();
+  m_pIface = std::move(pPushButton);
   CFWL_Widget::Initialize();
   return FWL_Error::Succeeded;
 }