Change nested navigations' fetches destinations.

Currently, top-level and nested navigations set a request destination of
"document" when performing the "process a navigate fetch" algorithm.
This patch follows whatwg/fetch#948, splitting "document" into "frame"
and "iframe" for nested navigation requests.

source

@@ -83604,11 +83604,26 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
    <var>browsingContext</var>'s <span>active document</span>'s <span>relevant settings
    object</span>'s <span data-x="concept-environment-id">id</span>.</p></li>
 
-   <li><p>If <var>browsingContext</var> is a <span>child browsing context</span> and the
-   <span>browsing context container</span> of <var>browsingContext</var> has a <span>browsing
-   context scope origin</span>, then set <var>request</var>'s <span
-   data-x="concept-request-origin">origin</span> to that <span>browsing context scope
-   origin</span>.</p></li>
+   <li>
+    <p>If <var>browsingContext</var> is a <span>child browsing context</span>:</p>
+
+    <ol>
+     <li>If the <span>browsing context container</span> of <var>browsingContext</var> has a
+     <span>browsing context scope origin</span>, then set <var>request</var>'s <span
+     data-x="concept-request-origin">origin</span> to that <span>browsing context scope
+     origin</span>.</p></li>
+
+     <li>If <var>browsingContext</var>'s <span>browsing context container</span> is an
+     <code>iframe</code> element, set <var>request</var>'s <span
+     data-x="concept-request-destination">destination</span> to "<code
+     data-x="">iframe</code>".</li>
+
+     <li>If <var>browsingContext</var>'s <span>browsing context container</span> is an
+     <code>frame</code> element, set <var>request</var>'s <span
+     data-x="concept-request-destination">destination</span> to "<code
+     data-x="">frame</code>".</li>
+    </ol>
+   </li>
 
    <li><p>Let <var>done</var> be false and <var>reservedEnvironment</var> be null.</p></li>