Make window.window/frames/self deal with no browsing context

Before this change these getters did not account for Window object's browsing context potentially being null, resulting in a conceptual crash.

Language from tc39/ecma262#702.

Tests: web-platform-tests/wpt#15720.

Helps with #4363.

source

@@ -3015,6 +3015,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://tc39.es/ecma262/#sec-execution-contexts">JavaScript execution context</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#execution-context-stack">JavaScript execution context stack</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#sec-code-realms">JavaScript realm</dfn></li>
+     <li><dfn data-x-href="https://tc39.es/ecma262/#sec-lexical-environments">EnvironmentRecord</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#sec-built-in-function-objects">NewTarget</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#running-execution-context">running JavaScript execution context</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#surrounding-agent">surrounding agent</dfn></li>
@@ -79756,9 +79757,10 @@ dictionary <dfn>WindowPostMessageOptions</dfn> : <span>PostMessageOptions</span>
   <span>browsing context</span>.</span></p>
 
   <p>The <dfn><code data-x="dom-window">window</code></dfn>, <dfn><code
-  data-x="dom-frames">frames</code></dfn>, and <dfn><code data-x="dom-self">self</code></dfn> IDL
-  attributes, on getting, must all return this <code>Window</code> object's <span>browsing
-  context</span>'s <code>WindowProxy</code> object.</p>
+  data-x="dom-frames">frames</code></dfn>, and <dfn><code data-x="dom-self">self</code></dfn>
+  attributes' getters must return this <code>Window</code> object's <span
+  data-x="concept-relevant-realm">relevant Realm</span>.[[GlobalEnv]]'s
+  <span>EnvironmentRecord</span>'s [[GlobalThisValue]].</p>
 
   <p>The <dfn><code data-x="dom-document">document</code></dfn> IDL attribute, on getting, must
   return this <code>Window</code> object's <span data-x="concept-document-window">associated