Replace NULL with nullptr in all C++ files.

webrtc/base/sigslot.cc

 // sigslot.h: Signal/Slot classes
 //
 // Written by Sarah Thompson (sarah@telergy.com) 2002.
 //
 // License: Public domain. You are free to use this code however you like, with
 // the proviso that the author takes on no responsibility or liability for any
 // use.
 
 #include "webrtc/base/sigslot.h"
 
 namespace sigslot {
 
 #ifdef _SIGSLOT_HAS_POSIX_THREADS
 
 multi_threaded_global::multi_threaded_global() {
-  pthread_mutex_init(get_mutex(), NULL);
+  pthread_mutex_init(get_mutex(), nullptr);
 }
 
 multi_threaded_global::multi_threaded_global(const multi_threaded_global&) {
 }
 
 multi_threaded_global::~multi_threaded_global() = default;
 
 void multi_threaded_global::lock() {
   pthread_mutex_lock(get_mutex());
 }
 
 void multi_threaded_global::unlock() {
   pthread_mutex_unlock(get_mutex());
 }
 
 multi_threaded_local::multi_threaded_local() {
-  pthread_mutex_init(&m_mutex, NULL);
+  pthread_mutex_init(&m_mutex, nullptr);
 }
 
 multi_threaded_local::multi_threaded_local(const multi_threaded_local&) {
-  pthread_mutex_init(&m_mutex, NULL);
+  pthread_mutex_init(&m_mutex, nullptr);
 }
 
 multi_threaded_local::~multi_threaded_local() {
   pthread_mutex_destroy(&m_mutex);
 }
 
 void multi_threaded_local::lock() {
   pthread_mutex_lock(&m_mutex);
 }
 
 void multi_threaded_local::unlock() {
   pthread_mutex_unlock(&m_mutex);
 }
 
 #endif  // _SIGSLOT_HAS_POSIX_THREADS
 
 };  // namespace sigslot