This would be another good one to look at. Layout test security/contentSecurityPolicy/video-with-data-url-allowed-by-media-src-star.html was broken by one of r203496 ("[GTK] Web view background colors don't work in accelerated compositing mode"), r203497 ("[GTK][Threaded Compositor] Web view background colors don't work"), or less-likely r203468 ("[Threaded Compositor] Flickering when zooming in/out in maps.google.com"). It's a bit tricky because the actual result is actually being displayed properly, but the expected result is now broken (it's a reftest). Note that it sometimes passes, which might be related to why we sometimes have a very high number of unexpected passes since enabling threaded compositor.
Fixed by r290443. Test expectations updated in https://trac.webkit.org/changeset/294140/webkit.