Fix bug in VTTCue::toJSON() found by new libcpp assertions. This is causing media/track/track-extended-descriptions.html to crash on some bots.
<rdar://104893184>
Pull request: https://github.com/WebKit/WebKit/pull/9425
Committed 259704@main (1a9f2a2c95a6): <https://commits.webkit.org/259704@main> Reviewed commits have been landed. Closing PR #9425 and removing active labels.