This is a flag that's used in the time marches on algorithm.

This introduces support for the TextTrackCue::Active flag. One place that isn't handled that is in the spec is when it says to unset the active flag of all the cues in a text track if that text track is removed from a media element, but no where do we have an API to remove a text track from a media element that I can see.

We've agreed for now that we won't be using TextTrackCue::active to be keeping track of the active text tracks. Instead we'll just be using cues whose start time and end time are valid for the current play back time as active. I'm leaving this bug open for now in case we reconsider in the near future.

I don't think we will be reconsidering in the near future. Using a sorted cue list and tracking the active cues via their start and end times is much more efficient for Gecko's solution.