Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/cpu_info.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.
  */
 
@@ skipping 21 matching lines @@
 
 #if defined(WEBRTC_WIN)
   SYSTEM_INFO si;
   GetSystemInfo(&si);
   number_of_cores = static_cast<int>(si.dwNumberOfProcessors);
 #elif defined(WEBRTC_LINUX) || defined(WEBRTC_ANDROID)
   number_of_cores = static_cast<int>(sysconf(_SC_NPROCESSORS_ONLN));
 #elif defined(WEBRTC_MAC)
   int name[] = {CTL_HW, HW_AVAILCPU};
   size_t size = sizeof(number_of_cores);
-  if (0 != sysctl(name, 2, &number_of_cores, &size, NULL, 0)) {
+  if (0 != sysctl(name, 2, &number_of_cores, &size, nullptr, 0)) {
     LOG(LS_ERROR) << "Failed to get number of cores";
     number_of_cores = 1;
   }
 #else
   LOG(LS_ERROR) << "No function to get number of cores";
 #endif
 
   LOG(LS_INFO) << "Available number of cores: " << number_of_cores;
 
   return number_of_cores;
 }
 }
 
 namespace webrtc {
 
 uint32_t CpuInfo::DetectNumberOfCores() {
   // Statically cache the number of system cores available since if the process
   // is running in a sandbox, we may only be able to read the value once (before
   // the sandbox is initialized) and not thereafter.
   // For more information see crbug.com/176522.
   static uint32_t logical_cpus = 0;
   if (!logical_cpus)
     logical_cpus = static_cast<uint32_t>(internal::DetectNumberOfCores());
   return logical_cpus;
 }
 
 }  // namespace webrtc