[WebCodecs] an encoder or decoder with being processed content should not be GCed
<rdar://problem/101533086>
Pull request: https://github.com/WebKit/WebKit/pull/5745
Committed 256007@main (856428a7241f): <https://commits.webkit.org/256007@main> Reviewed commits have been landed. Closing PR #5745 and removing active labels.