Raise "no such window" error in "dispatching actions" when browsing context (navigable) does no longer exist (#1861)

Author: whimboo

index.html

@@ -9161,6 +9161,9 @@ <h3>Dispatching actions</h3>
   <var>actions by tick</var>:
 
   <ol>
+   <li>If <var>browsing context</var> is <a>no longer open</a>, return
+    <a>error</a> with <a>error code</a> <a>no such window</a>.
+
    <li><p>Let <var>tick duration</var> be the result of <a>computing
     the tick duration</a> with argument <var>tick actions</var>.