Re: [webrtc-pc] Clarify "multiple sources of media stitched together" at Note describing replaceTrack (#2171)

@fippo The code at Chromium source appears to use 

`std::unique_ptr<WebRtcMediaStreamTrackAdapterMap::AdapterRef> track_ref;` at several occasions, particularly at https://github.com/chromium/chromium/blob/1dc524aa4005eb08c533f2a344d5812863ae60a5/content/renderer/media/webrtc/rtc_rtp_sender.cc#L192

```
    std::unique_ptr<WebRtcMediaStreamTrackAdapterMap::AdapterRef> track_ref;
    webrtc::MediaStreamTrackInterface* webrtc_track = nullptr;
    if (!with_track.IsNull()) {
      track_ref = track_map_->GetOrCreateLocalTrackAdapter(with_track);
      webrtc_track = track_ref->webrtc_track();
    }
    signaling_task_runner_->PostTask(
        FROM_HERE,
        base::BindOnce(
            &RTCRtpSender::RTCRtpSenderInternal::ReplaceTrackOnSignalingThread,
            this, std::move(track_ref), base::Unretained(webrtc_track),
            std::move(callback)));
```

is that the basis of the code which performs the task of "multiple sources of media stitched together" at Chromium? 

If yes, then what exactly does the code do, technically? That is, what is the flowchart for that procedure outlined in the same manner that the procedure for `replaceTrack()` is outlined in the specification at the first link at first comment?

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