Use runtime setting to enable ExtensionKit assertions. This setting is not enabled by default.
<rdar://problem/117997757>
Pull request: https://github.com/WebKit/WebKit/pull/20035
Committed 270318@main (0df974645cc9): <https://commits.webkit.org/270318@main> Reviewed commits have been landed. Closing PR #20035 and removing active labels.