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

@@ -31087,7 +31087,8 @@ interface <dfn>HTMLEmbedElement</dfn> : <span>HTMLElement</span> {
      document</span>'s <span>relevant settings object</span>, <span
      data-x="concept-request-destination">destination</span> is "<code data-x="">embed</code>",
      <span data-x="concept-request-credentials-mode">credentials mode</span> is "<code
-     data-x="">include</code>", and whose <span>use-URL-credentials flag</span> is set.</p></li>
+     data-x="">include</code>", <span data-x="concept-request-mode">mode</span> is "<code
+     data-x="">navigate</code>", and whose <span>use-URL-credentials flag</span> is set.</p></li>
 
      <li>
       <p><span data-x="concept-fetch">Fetch</span> <var>request</var>.</p>
@@ -31573,7 +31574,8 @@ interface <dfn>HTMLObjectElement</dfn> : <span>HTMLElement</span> {
      document</span>'s <span>relevant settings object</span>, <span
      data-x="concept-request-destination">destination</span> is "<code data-x="">object</code>",
      <span data-x="concept-request-credentials-mode">credentials mode</span> is "<code
-     data-x="">include</code>", and whose <span>use-URL-credentials flag</span> is set.</p></li>
+     data-x="">include</code>", <span data-x="concept-request-mode">mode</span> is "<code
+     data-x="">navigate</code>", and whose <span>use-URL-credentials flag</span> is set.</p></li>
 
      <li>
 
@@ -84511,10 +84513,21 @@ 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>'s <span data-x="bc-container">container</span> is non-null
-   and 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>'s <span data-x="bc-container">container</span> is non-null:</p>
+
+    <ol>
+     <li><p>If the <var>browsingContext</var>'s <span data-x="bc-container">container</span> 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>Set <var>request</var>'s <span
+     data-x="concept-request-destination">destination</span> to <var>browsingContext</var>'s
+     <span data-x="bc-container">container</span>'s <span data-x="concept-element-local-name">local
+     name</span>.</p></li>
+    </ol>
+   </li>
 
    <li><p>Let <var>reservedEnvironment</var> be null.</p></li>