This is needed to allow embedded launches in Safari, which haven't worked for a while now.

• There is no cookie partitioning support in Webkit and there hasn’t been for some time. See https://webkit.org/blog/8613/intelligent-tracking-prevention-2-1/
• 3rd party cookie use requires use of the Storage Access API. See https://webkit.org/blog/11545/updates-to-the-storage-access-api/ 

See https://movementforanopenweb.com/related-website-sets-multi-brand-limits/ for an example flow, including taking user through tool site as first-party

This may be a candidate to backport, but we'll see once the solution is here.