It has been timing out regularly on the bots for several days: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=svg%2Fas-object%2Fnested-embedded-svg-size-changes.html
Looking at NRWT output, it tooks above 12 seconds to run which is why it timed out. As the release bots are having no issue, I will mark it as SLOW in debug and see if it helps.
It was a mistake, the test is very slow (and still timing out enough that it doesn't warrant marking it as such).
See bug 77183.