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

webrtc/audio/audio_receive_stream.cc

Index: webrtc/audio/audio_receive_stream.cc
diff --git a/webrtc/audio/audio_receive_stream.cc b/webrtc/audio/audio_receive_stream.cc
index 9bc9468336bd344faa2f2fbecf0d2fffaf692998..8dc14e850f74f73f1fe5b59208bbe424962016e8 100644
--- a/webrtc/audio/audio_receive_stream.cc
+++ b/webrtc/audio/audio_receive_stream.cc
@@ -51,7 +51,7 @@ std::string AudioReceiveStream::Config::ToString() const {
   std::stringstream ss;
   ss << "{rtp: " << rtp.ToString();
   ss << ", rtcp_send_transport: "
-     << (rtcp_send_transport ? "(Transport)" : "nullptr");
+     << (rtcp_send_transport ? "(Transport)" : "null");
   ss << ", voe_channel_id: " << voe_channel_id;
   if (!sync_group.empty()) {
     ss << ", sync_group: " << sync_group;