Replace NULL with nullptr in all C++ files.

webrtc/pc/remoteaudiosource.cc

Index: webrtc/pc/remoteaudiosource.cc
diff --git a/webrtc/pc/remoteaudiosource.cc b/webrtc/pc/remoteaudiosource.cc
index b2568191bda155567ac8038e50d194616047974d..bf356f4ae73e634630a89b8fd0d9e3f69f8318ec 100644
--- a/webrtc/pc/remoteaudiosource.cc
+++ b/webrtc/pc/remoteaudiosource.cc
@@ -102,14 +102,14 @@ void RemoteAudioSource::SetVolume(double volume) {
 }
 
 void RemoteAudioSource::RegisterAudioObserver(AudioObserver* observer) {
-  RTC_DCHECK(observer != NULL);
+  RTC_DCHECK(observer != nullptr);
   RTC_DCHECK(std::find(audio_observers_.begin(), audio_observers_.end(),
                        observer) == audio_observers_.end());
   audio_observers_.push_back(observer);
 }
 
 void RemoteAudioSource::UnregisterAudioObserver(AudioObserver* observer) {
-  RTC_DCHECK(observer != NULL);
+  RTC_DCHECK(observer != nullptr);
   audio_observers_.remove(observer);
 }