Disallow window.open() during unload

Tests: web-platform-tests/wpt#12577

Author: TimothyGu

source

@@ -78278,6 +78278,8 @@ dictionary <dfn>WindowPostMessageOptions</dfn> : <span>PostMessageOptions</span>
   and a string <var>features</var>, are as follows:</p>
 
   <ol>
+   <li><p>If the <span>event loop</span>'s <span>termination nesting level</span> is nonzero, return
+   null.</p></li>
 
    <li><p>Let <var>entry settings</var> be the <span>entry settings object</span>.</p></li>