Replace NULL with nullptr in all C++ files.

webrtc/modules/remote_bitrate_estimator/test/estimators/nada.cc

Index: webrtc/modules/remote_bitrate_estimator/test/estimators/nada.cc
diff --git a/webrtc/modules/remote_bitrate_estimator/test/estimators/nada.cc b/webrtc/modules/remote_bitrate_estimator/test/estimators/nada.cc
index 39cad74ce4a79d4bce2c1a85a8115252a01d283e..ee7fd4b8ed7bfa55b4887f6b5aac7e73920a6bed 100644
--- a/webrtc/modules/remote_bitrate_estimator/test/estimators/nada.cc
+++ b/webrtc/modules/remote_bitrate_estimator/test/estimators/nada.cc
@@ -99,7 +99,7 @@ FeedbackPacket* NadaBweReceiver::GetFeedback(int64_t now_ms) {
   const int64_t kPacketLossPenaltyMs = 1000;  // Referred as d_L.
 
   if (now_ms - last_feedback_ms_ < 100) {
-    return NULL;
+    return nullptr;
   }
 
   float loss_fraction = RecentPacketLossRatio();