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>Let <var>forNavigation</var> be true if <a for=request>mode</a> is
+     <code>navigate</code>; false otherwise.
+     <li><p>Return the result of running <a>HTTP fetch</a> given <var>fetchParams</var> and
+     <var>forNavigation</var>.
+    </ol>
   </dl>
 
  <li><p>Return a <a for=/>network error</a>.
@@ -5118,7 +5124,7 @@ steps:
 
 <div algorithm>
 <p>To <dfn export id=concept-http-fetch>HTTP fetch</dfn>, given a <a for=/>fetch params</a>
-<var>fetchParams</var> and an optional boolean <var>makeCORSPreflight</var> (default false), run
+<var>fetchParams</var> and an optional boolean <var>allowCORS</var> (default false), run
 these steps:
 <!-- This is exported for service workers, but that specification only mentions it in passing. -->
 
@@ -5218,7 +5224,7 @@ these steps:
 
   <ol>
    <li>
-    <p>If <var>makeCORSPreflight</var> is true and one of these conditions is true:
+    <p>If <var>allowCORS</var> is true and one of these conditions is true:
 
     <ul class=brief>
      <li><p>There is no <a>method cache entry match</a> for <var>request</var>'s