Add attribute changed steps to dialog element

When the open attribute is removed from the dialog element we remove the element from the document's open dialogs list.

source

@@ -61801,6 +61801,21 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
    data-x="attr-hidden">hidden</code> attribute to hide it.</p>
   </div>
 
+  <p>The following <span data-x="concept-element-attributes-change-ext">attribute change
+  steps</span>, given <var>element</var>, <var>localName</var>, <var>oldValue</var>,
+  <var>value</var>, and <var>namespace</var> are used for <code>dialog</code> elements:</p>
+
+    <ol>
+     <li><p>If <var>namespace</var> is not null, then return.</p></li>
+
+     <li><p>If <var>localName</var> is not <code data-x="attr-dialog-open">open</code>, then
+     return.</p></li>
+
+     <li><p>If <var>value</var> is null, then <span data-x="list remove">remove</span>
+     <var>element</var> from <var>element</var>'s <span>node document</span>'s <span>open dialogs
+     list</span>.</p></p></li>
+    </ol>
+
   <p>The <code data-x="attr-tabindex">tabindex</code> attribute must not be specified on
   <code>dialog</code> elements.</p>