Restrict background fetch loads to HTTP family
<rdar://problem/106026139>
Pull request: https://github.com/WebKit/WebKit/pull/10792
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/38742
Committed 260994@main (39186fb707af): <https://commits.webkit.org/260994@main> Reviewed commits have been landed. Closing PR #10792 and removing active labels.