Make crossOriginObject.then undefined for promises

This allows promises to work better with cross-origin WindowProxy and Location objects.

Fixes whatwg/dom#536.

source

@@ -77856,13 +77856,6 @@ console.assert(iframeWindow.frameElement === null);
   caller needs to throw a <span>"<code>SecurityError</code>"</span> <code>DOMException</code>.</p>
 
   <ol>
-   <li><p>If <var>P</var> is <span>@@toStringTag</span>, <span>@@hasInstance</span>, or
-   <span>@@isConcatSpreadable</span>, then return <span>PropertyDescriptor</span>{
-   [[Value]]: undefined,
-   [[Writable]]: false,
-   [[Enumerable]]: false,
-   [[Configurable]]: true }.</p></li>
-
    <li><p>Let <var>crossOriginKey</var> be a tuple consisting of the <span>current settings
    object</span>, <var>O</var>'s <span>relevant settings object</span>, and <var>P</var>.</p></li>
 
@@ -77938,6 +77931,14 @@ console.assert(iframeWindow.frameElement === null);
     </ol>
    </li>
 
+   <li><p>If <var>P</var> is "<code data-x="">then</code>", <span>@@toStringTag</span>,
+   <span>@@hasInstance</span>, or <span>@@isConcatSpreadable</span>, then return
+   <span>PropertyDescriptor</span>{
+   [[Value]]: undefined,
+   [[Writable]]: false,
+   [[Enumerable]]: false,
+   [[Configurable]]: true }.</p></li>
+
    <li><p>Return undefined.</p></li>
   </ol>
 
@@ -78010,6 +78011,10 @@ console.assert(iframeWindow.frameElement === null);
     </ol>
    </li>
 
+   <li><p>If <var>keys</var> <span data-x="list contains">does not contain</span> "<code
+   data-x="">then</code>", then <span data-x="list append">append</span> "<code
+   data-x="">then</code>" to <var>keys</var>.</p></li>
+
    <li><p>Return the concatenation of <var>keys</var> and « <span>@@toStringTag</span>,
    <span>@@hasInstance</span>, <span>@@isConcatSpreadable</span> ».</p></li>
   </ol>