Add opaque "layout item" to widgets for caller's use.

xfa/fwl/core/fwl_widgetimp.h

Index: xfa/fwl/core/fwl_widgetimp.h
diff --git a/xfa/fwl/core/fwl_widgetimp.h b/xfa/fwl/core/fwl_widgetimp.h
index 3fdef9786c8ea39d24bf29a6b324f2b771222ffb..8a887126e35417727fb92bedea351a002ec6b772 100644
--- a/xfa/fwl/core/fwl_widgetimp.h
+++ b/xfa/fwl/core/fwl_widgetimp.h
@@ -80,6 +80,8 @@ class CFWL_WidgetImp {
   CFX_SizeF GetOffsetFromParent(IFWL_Widget* pParent);
   uint32_t GetEventKey() const;
   void SetEventKey(uint32_t key);
+  void* GetLayoutItem() const;
+  void SetLayoutItem(void* pItem);
 
  protected:
   friend class CFWL_WidgetImpDelegate;
@@ -162,6 +164,7 @@ class CFWL_WidgetImp {
   IFWL_WidgetDelegate* m_pCurDelegate;
   IFWL_Widget* m_pOuter;
   IFWL_Widget* m_pInterface;
+  void* m_pLayoutItem;
   int32_t m_iLock;
   uint32_t m_nEventKey;
 };