It seems playbin2 doesn't send all state-change messages in some cases. Not sure yet but it might be a GStreamer bug. For now removing the filter I commited in r56009 should fix media/event-attributes.html.
I'll reopen Bug 36115 too.
Oh well reverting r56009 makes the test flakey instead of failing 100% of the time... I'm working on a different patch related to seeking which seems to fix the test reliably. I'll do more testing and send it as part of another patch.