This new policy matches the Safari behavior when ITP is disabled and third-party cookies are blocked. The SOUP_COOKIE_JAR_ACCEPT_NO_THIRD_PARTY policy does not allow subresources to set cookies unless they match the domain of the main resource. The new policy makes an exception for domains that have been previously visited.
This patch was written by Michael Catanzaro, but it changed the behavior of SOUP_COOKIE_JAR_ACCEPT_NO_THIRD_PARTY. I just updated it to add a new policy instead.