Replace NULL with nullptr in all C++ files.

webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc

Index: webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc
diff --git a/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc b/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc
index b77550f12120ef55a3309c9f0e71d4c3542440a3..aceb3e979f9bf32a89d524eb93eb0caae49a47bd 100644
--- a/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc
+++ b/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc
@@ -60,7 +60,7 @@ int64_t RtpStream::GenerateFrame(int64_t time_now_us, PacketList* packets) {
   if (time_now_us < next_rtp_time_) {
     return next_rtp_time_;
   }
-  assert(packets != NULL);
+  assert(packets != nullptr);
   size_t bits_per_frame = (bitrate_bps_ + fps_ / 2) / fps_;
   size_t n_packets =
       std::max<size_t>((bits_per_frame + 4 * kMtu) / (8 * kMtu), 1u);
@@ -86,7 +86,7 @@ int64_t RtpStream::next_rtp_time() const {
 // Generates an RTCP packet.
 RtpStream::RtcpPacket* RtpStream::Rtcp(int64_t time_now_us) {
   if (time_now_us < next_rtcp_time_) {
-    return NULL;
+    return nullptr;
   }
   RtcpPacket* rtcp = new RtcpPacket;
   int64_t send_time_us = time_now_us + kSendSideOffsetUs;
@@ -171,7 +171,7 @@ void StreamGenerator::set_rtp_timestamp_offset(uint32_t ssrc, uint32_t offset) {
 // it possible to simulate different types of channels.
 int64_t StreamGenerator::GenerateFrame(RtpStream::PacketList* packets,
                                        int64_t time_now_us) {
-  assert(packets != NULL);
+  assert(packets != nullptr);
   assert(packets->empty());
   assert(capacity_ > 0);
   StreamMap::iterator it = std::min_element(streams_.begin(), streams_.end(),