Fix browsing context checks for Switch To Parent Frame when already the top-level browsing context.

Author: whimboo

index.html

@@ -3573,6 +3573,16 @@ <h3><dfn>Switch To Parent Frame</dfn></h3>
 <p>The <a>remote end steps</a> are:
 
 <ol>
+ <li><p>Run the substeps if the <a>current browsing context</a> is already the
+  <a>top-level browsing context</a>:
+
+  <ol>
+    <li><p>If the <a>current browsing context</a> is <a>no longer
+  open</a>, return <a>error</a> with <a>error code</a> <a>no such
+  window</a>.
+    <li><p>Return <a>success</a> with data <a><code>null</code></a>.
+  </ol>
+
  <li><p>If the <a>current parent browsing context</a> is <a>no longer
   open</a>, return <a>error</a> with <a>error code</a> <a>no such
   window</a>.