Replace NULL with nullptr or null in webrtc/audio/ and common_audio/.

webrtc/audio/audio_receive_stream_unittest.cc

Index: webrtc/audio/audio_receive_stream_unittest.cc
diff --git a/webrtc/audio/audio_receive_stream_unittest.cc b/webrtc/audio/audio_receive_stream_unittest.cc
index b28c62a6279b544e088a6c060afc4438a0bbb7d6..e6b81a8bab62d1de1f07f816834ae2a8971717b6 100644
--- a/webrtc/audio/audio_receive_stream_unittest.cc
+++ b/webrtc/audio/audio_receive_stream_unittest.cc
@@ -225,7 +225,7 @@ TEST(AudioReceiveStreamTest, ConfigToString) {
       "{rtp: {remote_ssrc: 1234, local_ssrc: 5678, transport_cc: off, nack: "
       "{rtp_history_ms: 0}, extensions: [{uri: "
       "urn:ietf:params:rtp-hdrext:ssrc-audio-level, id: 3}]}, "
-      "rtcp_send_transport: nullptr, voe_channel_id: 2}",
+      "rtcp_send_transport: null, voe_channel_id: 2}",
       config.ToString());
 }