XML documents end up with a unique origin in WebKit only. They have a regular origin in Blink and Firefox.
Created attachment 434143 [details] Patch
Created attachment 434210 [details] Patch
Committed r280300 (239953@main): <https://commits.webkit.org/239953@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 434210 [details].
<rdar://problem/81122007>