Steps 4-10 of https://www.w3.org/TR/webvtt1/#cue-computed-line
This part is harder to fit into our code structure because it depends on
how many cues are simultaneously visible - so it has to go in
WebvttSubtitle not WebvttCueParser (which only deals with individual
cues in isolation).
This removes the `isNormal()` method that was trying to approximate
the correct behaviour.
PiperOrigin-RevId: 309021686
