r58107 appears to expose a bug in QuickTime or QTKit that make http/tests/media/video-play-stall.html and http/tests/media/video-play-stall-seek.html fail on the Mac. Need to dig into QuickTime to see if there is a way to work around the problem, but we should skip the tests for now.
Tests skipped in http://trac.webkit.org/changeset/58109
Win slaves also hit this problem, I skipped the test in r58217.