Don't clear NowPlaying state unless the web process is registered as the NowPlaying app
<rdar://problem/62249870>
Created attachment 399369 [details] Patch
Created attachment 399378 [details] Patch
Created attachment 399392 [details] Patch
Committed r261709: <https://trac.webkit.org/changeset/261709> All reviewed patches have been landed. Closing bug and clearing flags on attachment 399392 [details].