Remove NULL in xfa/

This CL converts all NULL's to nullptr. All instances of comparison to nullptr
have been removed.

Review-Url: https://codereview.chromium.org/2095653002
diff --git a/xfa/fxfa/include/xfa_ffwidgethandler.h b/xfa/fxfa/include/xfa_ffwidgethandler.h
index 4aa86d2..1cd28ce 100644
--- a/xfa/fxfa/include/xfa_ffwidgethandler.h
+++ b/xfa/fxfa/include/xfa_ffwidgethandler.h
@@ -23,7 +23,7 @@
 
   CXFA_FFWidget* CreateWidget(CXFA_FFWidget* hParent,
                               XFA_WIDGETTYPE eType,
-                              CXFA_FFWidget* hBefore = NULL);
+                              CXFA_FFWidget* hBefore = nullptr);
 
   FX_BOOL OnMouseEnter(CXFA_FFWidget* hWidget);
   FX_BOOL OnMouseExit(CXFA_FFWidget* hWidget);
@@ -111,7 +111,7 @@
                             CXFA_Node* pBefore) const;
   CXFA_Node* CreateCopyNode(XFA_Element eElement,
                             CXFA_Node* pParent,
-                            CXFA_Node* pBefore = NULL) const;
+                            CXFA_Node* pBefore = nullptr) const;
   CXFA_Node* CreateTemplateNode(XFA_Element eElement,
                                 CXFA_Node* pParent,
                                 CXFA_Node* pBefore) const;