Replace NULL with nullptr in all C++ files.

webrtc/modules/rtp_rtcp/test/testAPI/test_api.cc

Index: webrtc/modules/rtp_rtcp/test/testAPI/test_api.cc
diff --git a/webrtc/modules/rtp_rtcp/test/testAPI/test_api.cc b/webrtc/modules/rtp_rtcp/test/testAPI/test_api.cc
index 7c7db4dabc7e936a8ad7ee4b6fad16ecba031dbe..216e9760d6e12fdf2900a1e1f69764cd242b3c66 100644
--- a/webrtc/modules/rtp_rtcp/test/testAPI/test_api.cc
+++ b/webrtc/modules/rtp_rtcp/test/testAPI/test_api.cc
@@ -103,7 +103,7 @@ class RtpRtcpAPITest : public ::testing::Test {
     module_.reset(RtpRtcp::CreateRtpRtcp(configuration));
     rtp_payload_registry_.reset(new RTPPayloadRegistry());
     rtp_receiver_.reset(RtpReceiver::CreateAudioReceiver(
-        &fake_clock_, NULL, NULL, rtp_payload_registry_.get()));
+        &fake_clock_, nullptr, nullptr, rtp_payload_registry_.get()));
   }
 
   std::unique_ptr<RTPPayloadRegistry> rtp_payload_registry_;