Change install location of WebKit extension processes
<rdar://problem/118590395>
Pull request: https://github.com/WebKit/WebKit/pull/20744
Committed 271152@main (e58b121f9feb): <https://commits.webkit.org/271152@main> Reviewed commits have been landed. Closing PR #20744 and removing active labels.
Re-opened since this is blocked by bug 265409