Send Touch Bar playback commands to MediaSession when appropriate.
<rdar://problem/74363311>
Created attachment 420366 [details] Patch
Created attachment 420370 [details] Patch
Created attachment 420371 [details] Patch
Committed r272890: <https://commits.webkit.org/r272890> All reviewed patches have been landed. Closing bug and clearing flags on attachment 420371 [details].