Pass a top-level navigation initiator origin to Fetch

To un-logjam the cookie layering work, I've started whatwg/fetch#1807.
That depends on this info to be piped into Fetch so we can
actually specify in WHATWG what SameSite=Strict means.

This patch plumbs that through on top-level navigatable fetches.

This doesn't build because it relies upon the corresponding
patch in Fetch. Let me know to land these.

source

@@ -102636,6 +102636,11 @@ location.href = '#foo';</code></pre>
      <dt><span data-x="concept-request-referrer-policy">referrer policy</span></dt>
      <dd><var>entry</var>'s <span data-x="she-document-state">document state</span>'s <span
      data-x="document-state-request-referrer-policy">request referrer policy</span></dd>
+
+     <dt><span data-x="concept-request-navigation-initiator-origin">navigation initiator origin</span></dt>
+     <dd><var>entry</var>'s <span data-x="she-document-state">document state</span>'s <span
+     data-x="document-state-initiator-origin">initiator origin</span> if <var>navigable</var> is a <span>top-level traversable</span>,
+     null otherwise.</dd>
     </dl>
    </li>