Make window.name deal with lack of browsing context

Tests: web-platform-tests/wpt#15391.

Helps with #4363.

source

@@ -78750,10 +78750,27 @@ dictionary <dfn>WindowPostMessageOptions</dfn> : <span>PostMessageOptions</span>
 
   <hr>
 
-  <p>The <dfn><code data-x="dom-name">name</code></dfn> attribute of the <code>Window</code> object
-  must, on getting, return the current <span data-x="browsing context name">name</span> of the
-  <span>browsing context</span>; and, on setting, set the <span data-x="browsing context
-  name">name</span> of the <span>browsing context</span> to the new value.</p>
+  <p>The <dfn><code data-x="dom-name">name</code></dfn> attribute's getter, when invoked, must run
+  these steps:</p>
+
+  <ol>
+   <li><p>If this <code>Window</code> object does not have an associated <span>browsing
+   context</span>, then return the empty string.</p></li>
+
+   <li><p>Return this <code>Window</code> object's associated <span>browsing context</span>'s <span
+   data-x="browsing context name">name</span>.</p></li>
+  </ol>
+
+  <p>The <code data-x="dom-name">name</code> attribute's setter, when invoked, must run these
+  steps:</p>
+
+  <ol>
+   <li><p>If this <code>Window</code> object does not have an associated <span>browsing
+   context</span>, then return.</p></li>
+
+   <li><p>Set this <code>Window</code> object's associated <span>browsing context</span>'s <span
+   data-x="browsing context name">name</span> to the given value.</p></li>
+  </ol>
 
   <p class="note">The name <a href="#resetBCName">gets reset</a> when the browsing context is
   <span data-x="navigate">navigated</span> to another <span>origin</span>.</p>