Prevent double-focusing with autofocus="" inside <dialog>

When the dialog focusing steps are called, we prevent further
autofocusing for the document. This fixes #4788.

Tests: web-platform-tests/wpt#19151

source

@@ -57860,6 +57860,18 @@ interface <dfn>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
 
    <li><p>Run the <span>focusing steps</span> for <var>control</var>.</p></li>
 
+   <li><p>Let <var>topDocument</var> be the <span>active document</span> of <var>control</var>'s
+   <span>node document</span>'s <span data-x="concept-document-bc">browsing context</span>'s
+   <span>top-level browsing context</span>.</p></li>
+
+   <li><p>If <var>control</var>'s <span>node document</span>'s <span>origin</span> is not the
+   <span data-x="same origin">same</span> as the <span>origin</span> of <var>topDocument</var>,
+   then return.</p></li>
+
+   <li><p><span data-x="list empty">Empty</span> <var>topDocument</var>'s
+   <span>autofocus candidates</span>.</p></li>
+
+   <li><p>Set <var>topDocument</var>'s <span>autofocus processed flag</span> to true.</p></li>
   </ol>
 
   <p>If at any time a <code>dialog</code> element is <span data-x="node is removed from a