Tighten webpushd sandbox on iOS by changing the default policy to deny.
<rdar://problem/103494644>
Pull request: https://github.com/WebKit/WebKit/pull/8736
Committed 259014@main (f8504aca7460): <https://commits.webkit.org/259014@main> Reviewed commits have been landed. Closing PR #8736 and removing active labels.