stepSubtitleDelay
Shifts the subtitle delay so the line offset lines away starts now (#491), for subtitles that are out of sync by a line or so: +1 brings the next line forward to now, -1 holds the previous one back to now, and 0 starts the line showing now. The result is the new delay, published as PlayerSnapshot.subtitleDelay like a setSubtitleDelay.
Throws
when no subtitle track is selected, when there is no such line, or when the delay it needs is more than DELAY_MAX either way.