Reader background is lost after multitasking.
<rdar://problem/62941837>
Created attachment 398659 [details] Patch
Committed r261252: <https://trac.webkit.org/changeset/261252> All reviewed patches have been landed. Closing bug and clearing flags on attachment 398659 [details].
Reverted r261252 for reason: Reland r260684 now that a proper fix has landed in Reader Committed r261364: <https://trac.webkit.org/changeset/261364>