[WebVTT] Cues with position setting containing aligment parse as 'auto'
<rdar://problem/111196313>
Pull request: https://github.com/WebKit/WebKit/pull/15222
Committed 265465@main (7798da0bec55): <https://commits.webkit.org/265465@main> Reviewed commits have been landed. Closing PR #15222 and removing active labels.