This appears to have started after <http://trac.webkit.org/changeset/178183> (bug 140298.)
Going to mark it as ImageOnlyFailure for now, since the result is not visually wrong, just has a tiny shading difference in some pixels.
Is this tiny shading difference really ok? Given what this change did, memory corruption sounds like the most likely cause for tiny differences. Can you explain it otherwise?
Looks like the rollout in r179839 fixed this on all bots.