Replace NULL with nullptr in all C++ files.

webrtc/test/win/run_loop_win.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/run_loop.h"
 
 #include <assert.h>
 
 #include <conio.h>
 #include <stdio.h>
 #include <Windows.h>
 
 namespace webrtc {
 namespace test {
 
 void PressEnterToContinue() {
   puts(">> Press ENTER to continue...");
 
   MSG msg;
   BOOL ret;
-  while ((ret = GetMessage(&msg, NULL, 0, 0)) != 0) {
+  while ((ret = GetMessage(&msg, nullptr, 0, 0)) != 0) {
     assert(ret != -1);
     TranslateMessage(&msg);
     DispatchMessage(&msg);
   }
 }
 }  // namespace test
 }  // namespace webrtc