This looks like it might be a recent regression? It looks like it clears up on a retry ... http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=fast%2Ffilesystem%2Fworkers%2Fasync-operations.html
After some consecutive timeouts at WebKit=r112344 and Chrome=r129323 now it looks ok? Will keep an eye a few more days.