it looks like this webgl compositing tests is failing on chromium macs (it's skipped on other macs).
Added to test_expectations.txt with http://trac.webkit.org/changeset/80618
Looks like at least some of those failing test have the same underlying cause as bug 56238.
I've started investigating this under Issue 82412 so am closing this as a duplicate of the other even though this one predated the other by a while.
*** This bug has been marked as a duplicate of bug 82412 ***