Bug 85143

For negative-length duration cues (which are always missed cues) update the implementation such that the 'exit' event is associated with the start time. "10. For each text track cue [...] in missed cues, prepare an event named exit for the TextTrackCue object with the later of the text track cue end time and the text track cue start time." For zero-length duration cues ensure both 'enter' and 'exit' events are dispatched and in order. "12. Finally, sort tasks in events that have the same time and same text track cue order by placing tasks that fire enter events before those that fire exit events." http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#the-track-element