Cross-origin WindowProxy's [[GetOwnProperty]] cannot return undefined

Callers do not anticipate it returning undefined in this case and browsers already throw.

Tests: will be added as part of https://bugzilla.mozilla.org/show_bug.cgi?id=1363208.

Fixes whatwg#4252.

source

@@ -79242,7 +79242,16 @@ interface <dfn>BarProp</dfn> {
        <!-- sort order: https://software.hixie.ch/utilities/js/live-dom-viewer/?saved=2860 -->
       </ol>
 
-     <li><p>If <var>value</var> is undefined, then return undefined.</p></li>
+     <li>
+      <p>If <var>value</var> is undefined, then:</p>
+
+      <ol>
+       <li><p>If ! <span>IsPlatformObjectSameOrigin</span>(<var>W</var>) is true, then return
+       undefined.</p></li>
+
+       <li><p>Throw a <span>"<code>SecurityError</code>"</span> <code>DOMException</code>.</p></li>
+      </ol>
+     </li>
 
      <li><p>Return <span>PropertyDescriptor</span>{
      [[Value]]: <var>value</var>,
@@ -81782,7 +81791,7 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
    <li><p>Let <var>property</var> be ! <span>CrossOriginGetOwnPropertyHelper</span>(<b>this</b>,
    <var>P</var>).</p></li>
 
-   <li><p>If <var>property</var> is not undefined, return <var>property</var>.</p></li>
+   <li><p>If <var>property</var> is not undefined, then return <var>property</var>.</p></li>
 
    <li><p>Throw a <span>"<code>SecurityError</code>"</span> <code>DOMException</code>.</p></li>
   </ol>