MediaSessionCoordinator should have join and leave methods
<rdar://problem/76021588>
Created attachment 424697 [details] WIP patch
Created attachment 424798 [details] Patch
Created attachment 424809 [details] Patch
Created attachment 424812 [details] Patch
Committed r275314: <https://commits.webkit.org/r275314> All reviewed patches have been landed. Closing bug and clearing flags on attachment 424812 [details].