Most likely due to disabled subpixel layout. Introduced in r145474 http://trac.webkit.org/changeset/145474 http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20-%20webkit.org&tests=fast%2Fbox-shadow%2Fbox-shadow-with-zero-radius.html
We don't have any platform specific expectation but we do have pixel tolerance: <meta name="fuzzy" content="maxDifference=0-64; totalPixels=0-626" /> Added by 255974@main. I am going to close this as 'RESOLVED CONFIGURATION CHANGED', if it still an issue, please reopen.