[WebVTT] Cues with line setting containing aligment parse as 'start'
<rdar://problem/111220055>
Pull request: https://github.com/WebKit/WebKit/pull/15240
Committed 265472@main (b2c434a9b456): <https://commits.webkit.org/265472@main> Reviewed commits have been landed. Closing PR #15240 and removing active labels.