Replace NULL with nullptr in all C++ files.

webrtc/modules/congestion_controller/delay_based_bwe_unittest_helper.cc

Index: webrtc/modules/congestion_controller/delay_based_bwe_unittest_helper.cc
diff --git a/webrtc/modules/congestion_controller/delay_based_bwe_unittest_helper.cc b/webrtc/modules/congestion_controller/delay_based_bwe_unittest_helper.cc
index 9aafc7b51607d551b982384761c616a5ca25c4a4..e90acbc84fc2bd7ced2a7fb95795b9ed2876f4b8 100644
--- a/webrtc/modules/congestion_controller/delay_based_bwe_unittest_helper.cc
+++ b/webrtc/modules/congestion_controller/delay_based_bwe_unittest_helper.cc
@@ -51,7 +51,7 @@ int64_t RtpStream::GenerateFrame(int64_t time_now_us,
   if (time_now_us < next_rtp_time_) {
     return next_rtp_time_;
   }
-  RTC_CHECK(packets != NULL);
+  RTC_CHECK(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);
@@ -126,7 +126,7 @@ void StreamGenerator::SetBitrateBps(int bitrate_bps) {
 // it possible to simulate different types of channels.
 int64_t StreamGenerator::GenerateFrame(std::vector<PacketInfo>* packets,
                                        int64_t time_now_us) {
-  RTC_CHECK(packets != NULL);
+  RTC_CHECK(packets != nullptr);
   RTC_CHECK(packets->empty());
   RTC_CHECK_GT(capacity_, 0);
   auto it =