Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/clock.cc

Index: webrtc/system_wrappers/source/clock.cc
diff --git a/webrtc/system_wrappers/source/clock.cc b/webrtc/system_wrappers/source/clock.cc
index f31556fc4ff38b1073877bdd2ea8ee264276dfec..22ec577368f4837387e335546290fa1992bdc0db 100644
--- a/webrtc/system_wrappers/source/clock.cc
+++ b/webrtc/system_wrappers/source/clock.cc
@@ -226,7 +226,7 @@ Clock* Clock::GetRealTimeClock() {
   static UnixRealTimeClock clock;
   return &clock;
 #else
-  return NULL;
+  return nullptr;
 #endif
 }