Replace NULL with nullptr in all C++ files.

webrtc/pc/mediastream.cc

Index: webrtc/pc/mediastream.cc
diff --git a/webrtc/pc/mediastream.cc b/webrtc/pc/mediastream.cc
index 18307923ac0694627efd2c2663369dcf79773182..740e11dcb14f5b83dd58dcda74c6d748c6f0f68e 100644
--- a/webrtc/pc/mediastream.cc
+++ b/webrtc/pc/mediastream.cc
@@ -57,7 +57,7 @@ rtc::scoped_refptr<AudioTrackInterface>
 MediaStream::FindAudioTrack(const std::string& track_id) {
   AudioTrackVector::iterator it = FindTrack(&audio_tracks_, track_id);
   if (it == audio_tracks_.end())
-    return NULL;
+    return nullptr;
   return *it;
 }
 
@@ -65,7 +65,7 @@ rtc::scoped_refptr<VideoTrackInterface>
 MediaStream::FindVideoTrack(const std::string& track_id) {
   VideoTrackVector::iterator it = FindTrack(&video_tracks_, track_id);
   if (it == video_tracks_.end())
-    return NULL;
+    return nullptr;
   return *it;
 }
 
@@ -82,7 +82,7 @@ bool MediaStream::AddTrack(TrackVector* tracks, Track* track) {
 template <typename TrackVector>
 bool MediaStream::RemoveTrack(TrackVector* tracks,
                               MediaStreamTrackInterface* track) {
-  RTC_DCHECK(tracks != NULL);
+  RTC_DCHECK(tracks != nullptr);
   if (!track)
     return false;
   typename TrackVector::iterator it = FindTrack(tracks, track->id());