Gardening: One more MacOS 10.6 Debug failure expectation

BUG=418091
TBR=eustas@chromium.org,eae@chromium.org

Review URL: https://codereview.chromium.org/625443003

git-svn-id: svn://svn.chromium.org/blink/trunk@183100 bbb929c8-8fbe-4397-9dbb-9b2b20218538