Move out of OnTrack/OnRemoveTrack libwebrtc callbacks
<rdar://problem/99481570>
Pull request: https://github.com/WebKit/WebKit/pull/3953
Committed 254128@main (704f5d6f10f8): <https://commits.webkit.org/254128@main> Reviewed commits have been landed. Closing PR #3953 and removing active labels.