It's flaky on all the Chromium bots: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#showExpectations=true&tests=fast%2Ftable%2Fmultiple-captions-display.xhtml. It's differently flaky on GTK (see bug 73772).
Marked LayoutTest bugs, bugs with Chromium IDs, and some others as WontFix. Test failure bugs still are trackable via TestExpectations or disabled unit tests.