SecItemShimProxy should be a MessageReceiver - It is a refcounted but singleton via WorkQueueMessageReceiver
<rdar://problem/99815679>
Pull request: https://github.com/WebKit/WebKit/pull/4253
Committed 254750@main (5b6e91526e2b): <https://commits.webkit.org/254750@main> Reviewed commits have been landed. Closing PR #4253 and removing active labels.