Allow legacy custom scheme registration on non-main threads
Created attachment 411221 [details] Patch
Note: we did something similar for WKURLSchemeTask in r247461 because people just want to do loading things on different threads.
Committed r268401: <https://trac.webkit.org/changeset/268401> All reviewed patches have been landed. Closing bug and clearing flags on attachment 411221 [details].
<rdar://problem/70259314>
Should we log an error when this happens, so the developer becomes aware?