See http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=compositing%2Fcheckerboard.html&showExpectations=true&group=%40ToT%20GPU%20-%20chromium.org The checkerboard pattern is not perfectly black and white (grays appear) in the lower right corner.
This may be related to bug 54409 (since that also shows diffs in the lower right corner)
I don't know when this test started to fail, since we were skipping it until r78619 (bug 54409 was caused by r78484, which happened before we unskipped it).
This is definitely related to bug 55409.
Committed r79084: <http://trac.webkit.org/changeset/79084>