RemoteRealtimeMediaSourceProxy::whenReady should support more than one callback
<rdar://problem/118709394>
Pull request: https://github.com/WebKit/WebKit/pull/20813
Committed 271073@main (e7ce0121a9b2): <https://commits.webkit.org/271073@main> Reviewed commits have been landed. Closing PR #20813 and removing active labels.