At least on Chromium debug builds. I suspect this is because some times the timeout is called before the events finish. This is likely failing for the same reason as bug 23633. Ever since bug 23633 has been landed, the other test has not failed.
Created attachment 27431 [details] Ups the timeout from 1 to 2
Simon or Chris should comment on this, I don't know what this test is about.
I landed a similar change for another layout test and that test is no longer flakey for us. You can see it here https://bugs.webkit.org/show_bug.cgi?id=23633 .
Comment on attachment 27431 [details] Ups the timeout from 1 to 2 r=me
Landed as http://trac.webkit.org/changeset/40874.