Prevent WebCodecs encoder and decoder from being GCed in case they are flushing
<rdar://problem/102360806>
Pull request: https://github.com/WebKit/WebKit/pull/6514
Committed 256739@main (688c513969e4): <https://commits.webkit.org/256739@main> Reviewed commits have been landed. Closing PR #6514 and removing active labels.