Use VoiceChannel/VideoChannel directly from RtpSender/RtpReceiver.

webrtc/api/rtpreceiverinterface.h

 /*
  *  Copyright 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 // This file contains interfaces for RtpReceivers
 // http://w3c.github.io/webrtc-pc/#rtcrtpreceiver-interface
 
 #ifndef WEBRTC_API_RTPRECEIVERINTERFACE_H_
 #define WEBRTC_API_RTPRECEIVERINTERFACE_H_
 
 #include <string>
 
 #include "webrtc/api/mediastreaminterface.h"
 #include "webrtc/api/proxy.h"
+#include "webrtc/api/rtpparameters.h"
 #include "webrtc/base/refcount.h"
 #include "webrtc/base/scoped_ref_ptr.h"
 #include "webrtc/pc/mediasession.h"
 
 namespace webrtc {
 
 class RtpReceiverObserverInterface {
  public:
+  // Note: Currently if there are multiple RtpReceivers of the same media type,
+  // they will all call OnFirstPacketReceived at once.
+  //
+  // In the future, it's likely that an RtpReceiver will only call
+  // OnFirstPacketReceived when a packet is received specifically for its
+  // SSRC/mid.
   virtual void OnFirstPacketReceived(cricket::MediaType media_type) = 0;
 
  protected:
   virtual ~RtpReceiverObserverInterface() {}
 };
 
 class RtpReceiverInterface : public rtc::RefCountInterface {
  public:
   virtual rtc::scoped_refptr<MediaStreamTrackInterface> track() const = 0;
 
+  // Audio or video receiver?
+  virtual cricket::MediaType media_type() const = 0;
+
   // Not to be confused with "mid", this is a field we can temporarily use
   // to uniquely identify a receiver until we implement Unified Plan SDP.
   virtual std::string id() const = 0;
 
   // The WebRTC specification only defines RTCRtpParameters in terms of senders,
   // but this API also applies them to receivers, similar to ORTC:
   // http://ortc.org/wp-content/uploads/2016/03/ortc.html#rtcrtpparameters*.
   virtual RtpParameters GetParameters() const = 0;
   virtual bool SetParameters(const RtpParameters& parameters) = 0;
 
+  // Does not take ownership of observer.
+  // Must call SetObserver(nullptr) before the observer is destroyed.
   virtual void SetObserver(RtpReceiverObserverInterface* observer) = 0;
 
-  virtual cricket::MediaType media_type() = 0;
-
  protected:
   virtual ~RtpReceiverInterface() {}
 };
 
 // Define proxy for RtpReceiverInterface.
 BEGIN_SIGNALING_PROXY_MAP(RtpReceiver)
 PROXY_CONSTMETHOD0(rtc::scoped_refptr<MediaStreamTrackInterface>, track)
+PROXY_CONSTMETHOD0(cricket::MediaType, media_type)
 PROXY_CONSTMETHOD0(std::string, id)
 PROXY_CONSTMETHOD0(RtpParameters, GetParameters);
 PROXY_METHOD1(bool, SetParameters, const RtpParameters&)
 PROXY_METHOD1(void, SetObserver, RtpReceiverObserverInterface*);
-PROXY_METHOD0(cricket::MediaType, media_type);
 END_SIGNALING_PROXY()
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_RTPRECEIVERINTERFACE_H_