[Cocoa] Thread-unsafe use of Vector in WebAVSampleBufferErrorListener
<rdar://problem/117950166>
Pull request: https://github.com/WebKit/WebKit/pull/20004
Committed 270237@main (66793be1c5e2): <https://commits.webkit.org/270237@main> Reviewed commits have been landed. Closing PR #20004 and removing active labels.