Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/rw_lock_posix.cc

 /*
  *  Copyright (c) 2011 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/system_wrappers/source/rw_lock_posix.h"
 
 namespace webrtc {
 
 RWLockPosix::RWLockPosix() : lock_() {
 }
 
 RWLockPosix::~RWLockPosix() {
   pthread_rwlock_destroy(&lock_);
 }
 
 RWLockPosix* RWLockPosix::Create() {
   RWLockPosix* ret_val = new RWLockPosix();
   if (!ret_val->Init()) {
     delete ret_val;
-    return NULL;
+    return nullptr;
   }
   return ret_val;
 }
 
 bool RWLockPosix::Init() {
   return pthread_rwlock_init(&lock_, 0) == 0;
 }
 
 void RWLockPosix::AcquireLockExclusive() {
   pthread_rwlock_wrlock(&lock_);
 }
 
 void RWLockPosix::ReleaseLockExclusive() {
   pthread_rwlock_unlock(&lock_);
 }
 
 void RWLockPosix::AcquireLockShared() {
   pthread_rwlock_rdlock(&lock_);
 }
 
 void RWLockPosix::ReleaseLockShared() {
   pthread_rwlock_unlock(&lock_);
 }
 
 }  // namespace webrtc