Quick authoring check shows that DragController::client() has never been used since its inception in <http://trac.webkit.org/r19039>. Moreover, it exposes DragController internals, which diminishes its encapsulation capabilities.
Created attachment 384787 [details] Patch
Comment on attachment 384787 [details] Patch Clearing flags on attachment: 384787 Committed r253094: <https://trac.webkit.org/changeset/253094>
All reviewed patches have been landed. Closing bug.
<rdar://problem/57615121>