Make Yosemite results the default, and special case Mavericks and Mountain Lion.
Created attachment 241445 [details] proposed fix
Committed <http://trac.webkit.org/r176042>.
Hmm, this somehow broke Mountain Lion and Mavericks. Rolled out in r176046, I'll investigate what's going on. May end up with a different approach.
The different approach is in bug 138729.