Currently we don't fully clean up the WebContent process with site isolation, but that should get fixed and then we can re-enable the http/tests/site-isolation/basic-iframe.html which times out. See: https://results.webkit.org/?suite=layout-tests&test=http%2Ftests%2Fsite-isolation%2Fbasic-iframe.html
<rdar://problem/110914967>
This was done for macOS in bug 260720, and bug 252504 tracks iOS flakiness. Duping to that. *** This bug has been marked as a duplicate of bug 252504 ***