Replace NULL with nullptr in all C++ files.

webrtc/base/sigslot.cc

Index: webrtc/base/sigslot.cc
diff --git a/webrtc/base/sigslot.cc b/webrtc/base/sigslot.cc
index 9628217dc0729b0847c97a4567650beae235afd8..bcdd11798ac95c26d7a499ebf9aad7523a8b21cf 100644
--- a/webrtc/base/sigslot.cc
+++ b/webrtc/base/sigslot.cc
@@ -13,7 +13,7 @@ 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&) {
@@ -30,11 +30,11 @@ void multi_threaded_global::unlock() {
 }
 
 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() {