[Cocoa] Adopt -[AVContentKey revoke] in WebAVContentKeyGroup
<rdar://problem/113405292>
Pull request: https://github.com/WebKit/WebKit/pull/19853
Committed 270096@main (177a218402b9): <https://commits.webkit.org/270096@main> Reviewed commits have been landed. Closing PR #19853 and removing active labels.