Revert to doing unloading steps *after* pagehide

This change reverts 386360c (#3957).
Specifically, it reorders two steps in the “unload a document” algorithm such
that the spec once again requires UAs to wait to run any “unloading document
visibility change steps” from other applicable specifications (in particular,
the Page Visibility spec) until *after* firing the pagehide event.

In practice, in combination with requirements in the Page Visibility spec,
this change has the effect of stating that UAs must wait to fire the
visibilitychange event until after the pagehide event has fired. That
ordering matches the behavior current implemented in all engines.

Otherwise, without this change, the ordering of firing for the events
doesn’t match what implementations actually do.

See also:
* w3c/page-visibility#39
* #5949 (comment)

source

@@ -85279,20 +85279,21 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
 
    <li><p>Set <var>document</var>'s <span>page showing</span> flag to false.</p></li>
 
-   <li>
-    <p>Run any <dfn>unloading document visibility change steps</dfn> for <var>document</var> that
-    are defined by <span>other applicable specifications</span>.</p>
-
-    <p class="note">This is specifically intended for use by <cite>Page Visibility</cite>. <ref spec=PAGEVIS></p>
-   </li>
-
    <li><p><span data-x="concept-event-fire">Fire an event</span> named <code
    data-x="event-pagehide">pagehide</code> at <var>document</var>'s <span>relevant global
    object</span>, using <code>PageTransitionEvent</code>, with the <code
    data-x="dom-PageTransitionEvent-persisted">persisted</code> attribute initialized to true if
    <var>document</var>'s <i data-x="concept-document-salvageable">salvageable</i> state is
    true, and false otherwise, and <var>legacy target override flag</var> set.</p></li>
 
+   <li>
+    <p>Run any <dfn>unloading document visibility change steps</dfn> for <var>document</var> that
+    are defined by <span>other applicable specifications</span>.</p>
+
+    <p class="note">This is specifically intended for use by <cite>Page Visibility</cite>.
+    <ref spec=PAGEVIS></p>
+   </li>
+
    <li><p><i>Unload event</i>: If <var>document</var>'s <span>fired unload</span> flag is
    false, then <span data-x="concept-event-fire">fire an event</span> named <code
    data-x="event-unload">unload</code> at <var>document</var>'s <span>relevant global object</span>,