Move VideoTrackSourceProxy creation into VideoTrack.

This CL contains a part of a previously reviewed, landed, reverted,
relanded and re-reverted CL:
https://webrtc-review.googlesource.com/c/src/+/250180

Bug: webrtc:13540
Change-Id: Id6df8da5ff61e3ec90d0b3f3d828e8f670d4931b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/251860
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36025}

pc/audio_track.h

@@ -21,6 +21,10 @@
 
 namespace webrtc {
 
+// TODO(tommi): Instead of inheriting from `MediaStreamTrack<>`, implement the
+// properties directly in this class. `MediaStreamTrack` doesn't guard against
+// conflicting access, so we'd need to override those methods anyway in this
+// class in order to make sure things are correctly checked.
 class AudioTrack : public MediaStreamTrack<AudioTrackInterface>,
                    public ObserverInterface {
  protected: