fetch and XHR should compute their origin from their context
<rdar://98313387>
<rdar://problem/107414766>
Pull request: https://github.com/WebKit/WebKit/pull/12169
Committed 262403@main (878b8e74ec2f): <https://commits.webkit.org/262403@main> Reviewed commits have been landed. Closing PR #12169 and removing active labels.