[MediaSession] Small refactoring of MediaSession implementation classes
Created attachment 425066 [details] Patch
Created attachment 425069 [details] Patch
Created attachment 425070 [details] Patch for landing
Committed r275458: <https://commits.webkit.org/r275458> All reviewed patches have been landed. Closing bug and clearing flags on attachment 425070 [details].
<rdar://problem/76243027>