Allow HTTP scheme fetches to make CORS preflight

This allows navigations to make CORS preflight requests, if the
navigation contains a request that is not safelisted. Navigation does
not (yet) provide APIs for non-safelisted requests; this update to the
fetch spec is a prerequisite for HTML spec changes that might add those
APIs (i.e. PUT method support in forms).

fetch.bs

@@ -5106,7 +5106,13 @@ steps:
     <p>When in doubt, return a <a>network error</a>.
 
    <dt><a>HTTP(S) scheme</a>
-   <dd><p>Return the result of running <a>HTTP fetch</a> given <var>fetchParams</var>.
+   <dd>
+    <ol>
+     <li><p>If <var>request</var>'s <a for=request>mode</a> is "<code>navigate</code>", then return
+     the result of running <a>HTTP fetch</a> given <var>fetchParams</var> and true.
+
+     <li><p>Return the result of running <a>HTTP fetch</a> given <var>fetchParams</var>.
+    </ol>
   </dl>
 
  <li><p>Return a <a for=/>network error</a>.