It was disabled in r243490 because bubblewrap sandbox needs a valid WebsiteDataStore before launching the web process. We can use the default WebsiteDataStore from the WebProcessPool and only use the prewarmed process later if the given WebsiteDataStore is the same as the WebProcessPool one.
Created attachment 371271 [details]
Comment on attachment 371271 [details]
Excellent! I had hoped something like this might work....
Committed r246101: <https://trac.webkit.org/changeset/246101>