Add a VideoTrackGenerator skeleton
<rdar://problem/120708226>
Pull request: https://github.com/WebKit/WebKit/pull/22542
Committed 272827@main (47a504ad8797): <https://commits.webkit.org/272827@main> Reviewed commits have been landed. Closing PR #22542 and removing active labels.
Re-opening for pull request https://github.com/webkit/webkit/pull/22578
Committed 272845@main (fb269e658112): <https://commits.webkit.org/272845@main> Reviewed commits have been landed. Closing PR #22578 and removing active labels.