[Cocoa] Refactor: update MediaPlayerPrivateAVFoundation's cached duration from duration KVO
<rdar://problem/118334048>
Pull request: https://github.com/WebKit/WebKit/pull/20418
Committed 270814@main (736ffded229d): <https://commits.webkit.org/270814@main> Reviewed commits have been landed. Closing PR #20418 and removing active labels.