The "rules for updating the text track rendering" are not run when mode changes from hidden to showing

Porting from https://www.w3.org/Bugs/Public/show_bug.cgi?id=28236, where @foolip says:

The "time marches on" algorithm runs the "rules for updating the text track rendering" only for "affected tracks", i.e. tracks where the the active cues have changed. This misses cues in tracks that have gone from hidden to showing. Per spec, those cues would only become visible when another cue in the same track changes its active state.

This has been worked around in Blink by also running the "rules for updating the text track rendering" when a track goes from hidden to showing. It could possibly be spec'd as part of the "time marches on" algorithm, but that would only work because the idea is that this algorithm is run all the time, where in an implementation you would only run it when you have reason to think that any cues have changed their active state.

Perhaps @foolip would like to work on a PR/tests? :)