Add DTLSTransport info into sender/receiver state.

This is in preparation for letting Chrome extract DTLSTransport
information after SLD/SRD instead of doing it on-demand.

Bug: chromium:907849
Change-Id: Iac6b174c98d3d14136e1fd25bce4a9292f6c8b41
Reviewed-on: https://webrtc-review.googlesource.com/c/116984
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26289}

api/rtp_sender_interface.cc

@@ -25,4 +25,9 @@ std::vector<RtpEncodingParameters> RtpSenderInterface::init_send_encodings()
   return {};
 }
 
+rtc::scoped_refptr<DtlsTransportInterface> RtpSenderInterface::dtls_transport()
+    const {
+  return nullptr;
+}
+
 }  // namespace webrtc