Bug#266329 started to use RemoteImageBufferSetProxy::flushFrontBufferAsync only for flushing RemoteImageBuffer commands for Windows port. This isn't an ideal solution. Add a new method to RemoteRenderingBackendProxy to flush commands.
Pull request: https://github.com/WebKit/WebKit/pull/22157
Committed 272509@main (e71f3f749df1): <https://commits.webkit.org/272509@main> Reviewed commits have been landed. Closing PR #22157 and removing active labels.
<rdar://problem/120200674>