There have been a large number of flaky tests that all started timing out around 8/18/16.
This is very likely caused by <http://trac.webkit.org/r204601>, even though there is no obvious relationship to the failing tests.
The failures are rare, but cross-referencing multiple tests and multiple bots, I found that there are no timeouts on these before r204601, and then there are 2-3 failures for each revision.
The timeout happens in the same worker that ran hls/hls-accessiblity-describes-video-menu.html, only 2-3 minutes later.
Ryan skipped hls/hls-accessiblity-describes-video-menu.html in r205124 to see if it helps.
Well, that did it. Still a mystery why this test causes timeouts later.