NULL -> nullptr under athena/

athena/content/shell/dialogs.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "athena/content/public/dialogs.h"
 #include "base/logging.h"
 
 namespace athena {
 
 content::ColorChooser* OpenColorChooser(
     content::WebContents* web_contents,
     SkColor initial_color,
     const std::vector<content::ColorSuggestion>& suggestions) {
   NOTIMPLEMENTED();
-  return NULL;
+  return nullptr;
 }
 
 void OpenFileChooser(content::WebContents* web_contents,
                      const content::FileChooserParams& params) {
   NOTIMPLEMENTED();
 }
 
 }  // namespace athena