This allows us to decouple implementation classes from the
MediaChannel class.
Bug: webrtc:13931
Change-Id: I22f166cac17c344f943a0382048e8086a193affa
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/307000
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40179}