Make sure we only prewarm lockdown mode WebProcesses when the feature is enabled system-wide. It is risky to prewarm non-lockdown mode processes in this case.
<rdar://problem/122317130>
<rdar://121627632>
Pull request: https://github.com/apple/WebKit/pull/1058
Pull request: https://github.com/WebKit/WebKit/pull/23874
Committed 274150@main (2f9ef5beef7d): <https://commits.webkit.org/274150@main> Reviewed commits have been landed. Closing PR #23874 and removing active labels.