in bug 161914, mechanism was added to notify the MediaPlayer that the track selection changed. However, it only implemented it for MediaSourcePrivateAVFObjC. For consistency, both Mock and GStreamer should do it.
<rdar://problem/118165705>
Pull request: https://github.com/WebKit/WebKit/pull/20219
Committed 270450@main (0499848710c3): <https://commits.webkit.org/270450@main> Reviewed commits have been landed. Closing PR #20219 and removing active labels.