Replace NULL with nullptr in all C++ files.

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

Index: webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc
diff --git a/webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc b/webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc
index 1917789b7c90898d3731e689d98308f2399a63d0..3ede796ef8369454c95ef8691d5b43b24a657861 100644
--- a/webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc
+++ b/webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc
@@ -108,7 +108,7 @@ void RembReceiver::ReceivePacket(int64_t arrival_time_ms,
 FeedbackPacket* RembReceiver::GetFeedback(int64_t now_ms) {
   BWE_TEST_LOGGING_CONTEXT("Remb");
   uint32_t estimated_bps = 0;
-  RembFeedback* feedback = NULL;
+  RembFeedback* feedback = nullptr;
   if (LatestEstimate(&estimated_bps)) {
     StatisticianMap statisticians = recv_stats_->GetActiveStatisticians();
     RTCPReportBlock report_block;