See bug 61894. I'm not sure why this set is needed or if it is still necessary for the mac port, but it can lead to lots of wasted memory on test cases like 61006. If this can be removed, great! Otherwise this should probably be moved to the WebKit layer and utilize some method to save memory, possibly by storing a more compact representation instead of the full URL string or by only caching some URLs.
<rdar://problem/9540459>