The following compositing tests, which use masks, are failing when run in DRT / NRWT (--platform chromium-gpu). Examination of the pixel output indicates that masks appear to not be implemented yet in Chromium's layer renderer.
The failing tests are: compositing/masks/masked-ancestor.html compositing/masks/multiple-masks.html compositing/masks/simple-composited-mask.html
Indeed, masks haven't been implemented yet: http://code.google.com/p/chromium/issues/detail?id=53049
Note that even though mask support was added with http://trac.webkit.org/changeset/76475, compositing/masks/multiple-masks.html still fails: See http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=compositing%2Fmasks%2Fmultiple-masks.html&showExpectations=true&group=%40ToT%20GPU%20-%20chromium.org The left and the right images are supposed to be identical. The left one (which is not composited) is rendered incorrectly (the mask does not appear to be applied correctly). This only happens on Windows and Linux, so I suspect a Skia issue.
The listed tests now all pass on windows and Linux. I believe the last fix came in with bug 66442. Marking this as fixed; if there are still problems I'm not aware of please reopen.