This new error message is emitted on fast/hidpi/image-set-without-specified-width.html, and possibly on other tests: https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&revision=189617&showExpectations=true&tests=fast%2Fhidpi%2Fimage-set-without-specified-width.html
Created attachment 261009 [details] Patch
http://trac.webkit.org/changeset/189626 If there are other tests that get this wrong, the fixes for them will be different. I'm going to try to change WKTR in a way that we can easily find any similar mistakes.