Replace NULL with nullptr in all C++ files.

webrtc/test/null_platform_renderer.cc

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/test/video_renderer.h"
 
 namespace webrtc {
 namespace test {
 
 VideoRenderer* VideoRenderer::CreatePlatformRenderer(const char* window_title,
                                                      size_t width,
                                                      size_t height) {
-  return NULL;
+  return nullptr;
 }
 }  // test
 }  // webrtc