This allows subclasses of MediaSendChannel and MediaReceiveChannel
to derive from MediaChannelUtil without promising to implement
the interfaces.
Bug: webrtc:13931
Change-Id: I998de7566b343032c83cd6e5419f49349f41035f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/307140
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40185}