Fix bogus assert for subframe navigations

Closes #7380.

source

@@ -83759,14 +83759,15 @@ interface <dfn interface>BarProp</dfn> {
   </ol>
 
   <p>To <dfn data-x="obtain-browsing-context-navigation">obtain a browsing context to use for a
-  navigation response</dfn>, given a <span data-x="browsing context">browsing contexts</span>
+  navigation response</dfn>, given a <span data-x="browsing context">browsing context</span>
   <var>browsingContext</var>, a <span>sandboxing flag set</span> <var>sandboxFlags</var>, a
   <span>cross-origin opener policy</span> <var>navigationCOOP</var>, and a <span
   data-x="coop-enforcement-result">cross-origin opener policy enforcement result</span>
   <var>coopEnforcementResult</var>:</p>
 
   <ol>
-   <li><p>Assert: <var>browsingContext</var> is a <span>top-level browsing context</span>.</p></li>
+   <li><p>If <var>browsingContext</var> is not a <span>top-level browsing context</span>,
+   return <var>browsingContext</var>.</p></li>
 
    <li>
     <p>If <var>coopEnforcementResult</var>'s <span data-x="coop-enforcement-bcg-switch">needs a