Migrate away from LegacyDownloadClient and instead handle System Previews directly. This will allow us to remove the horrible code in the download client, as well eventually allow 3rd-party engines to use ARQL.
<rdar://problem/108524222>
Pull request: https://github.com/WebKit/WebKit/pull/13171
Committed 263393@main (d55e3cae6649): <https://commits.webkit.org/263393@main> Reviewed commits have been landed. Closing PR #13171 and removing active labels.
This patch seems to have broken Apple internal builds.
Build fix attempted in https://commits.webkit.org/263403@main