Change subframe traverse navigate event timing

This fires navigate events for subframes during a traversal earlier, before the unload event and in parallel with any fetches that occur. This is more consistent with non-traverse navigate events, and with top-level traverse navigate events.

WICG/navigation-api#207 has some previous relevant discussion. This represents moving from that issue's "late" navigate events to "early-but-still-async". It turns out the complexity was not too bad, when we got around to it in Chromium, and was actually helpful in order to fix a bug where traverse navigate events weren't firing when pages were bfcached.

Author: domenic

source

@@ -100273,6 +100273,16 @@ location.href = '#foo';</code></pre>
      data-x="she-document-state">document state</span>'s <span
      data-x="document-state-origin">origin</span>.</p></li>
 
+     <li id="descendant-navigable-traversal-navigate-events"><p>If <var>navigable</var> is not
+     <var>traversable</var>, and <var>targetEntry</var> is not <var>navigable</var>'s <span
+     data-x="nav-current-history-entry">current session history entry</span>, and
+     <var>oldOrigin</var> is the <span data-x="same origin">same</span> as <var>navigable</var>'s
+     <span data-x="nav-current-history-entry">current session history entry</span>'s <span
+     data-x="she-document-state">document state</span>'s <span
+     data-x="document-state-origin">origin</span>, then <span>fire a traverse <code
+     data-x="event-navigate">navigate</code> event</span> given <var>targetEntry</var> and
+     <var>userInvolvementForNavigateEvents</var>.</p></li>
+
      <li>
       <p>If <var>targetEntry</var>'s <span data-x="she-document">document</span> is null, or
       <var>targetEntry</var>'s <span data-x="she-document-state">document state</span>'s <span
@@ -100525,17 +100535,6 @@ location.href = '#foo';</code></pre>
         </ol>
        </li>
 
-       <li id="descendant-navigable-traversal-navigate-events"><p>If <var>navigable</var> is not
-       <var>traversable</var>, and <var>targetEntry</var> is not <var>navigable</var>'s <span
-       data-x="nav-current-history-entry">current session history entry</span>, and
-       <var>targetEntry</var>'s <span data-x="she-document-state">document state</span>'s <span
-       data-x="document-state-origin">origin</span> is the <span data-x="same origin">same</span> as
-       <var>navigable</var>'s <span data-x="nav-current-history-entry">current session history
-       entry</span>'s <span data-x="she-document-state">document state</span>'s <span
-       data-x="document-state-origin">origin</span>, then <span>fire a traverse <code
-       data-x="event-navigate">navigate</code> event</span> given <var>targetEntry</var> and
-       <var>userInvolvementForNavigateEvents</var>.</p></li>
-
        <li><p>Let <var>updateDocument</var> be an algorithm step which performs <span>update
        document for history step application</span> given <var>targetEntry</var>'s <span
        data-x="she-document">document</span>, <var>targetEntry</var>,