SECURE: Use the getter's global

The incumbent settings object will be incorrect if/when the getter is
called outside its own context.

A second stab at #263.

specs/powerfulfeatures/index.html

@@ -1218,8 +1218,8 @@ <h2 class="heading settled" data-level="5" id="detection"><span class="secno">5.
     <p>Both <code class="idl"><a data-link-type="idl" href="http://www.w3.org/TR/dom/#interface-window">Window</a></code>'s <code class="idl"><a data-link-type="idl" href="#dom-window-issecurecontext">isSecureContext</a></code> and <code class="idl"><a data-link-type="idl" href="https://html.spec.whatwg.org/multipage/workers.html#workerglobalscope">WorkerGlobalScope</a></code>'s
   <code class="idl"><a data-link-type="idl" href="#dom-workerglobalscope-issecurecontext">isSecureContext</a></code> attribute’s getters return
   <code>true</code> if <a href="#settings-secure">§6.1 Is settings object a secure context?</a> returns <code>Secure</code> when
-  executed on the <a data-link-type="dfn" href="http://www.w3.org/TR/html5/webappapis.html#incumbent-settings-object">incumbent settings object</a>, and <code>false</code>
-  otherwise.</p>
+  executed on the <a data-link-type="dfn" href="http://www.w3.org/TR/html5/webappapis.html#relevant-settings-object-for-a-global-object">relevant settings object</a> for the getter’s <a data-link-type="dfn" href="http://www.w3.org/TR/html5/webappapis.html#global-object">global
+  object</a>, and <code>false</code> otherwise.</p>
 </section>
 
 

specs/powerfulfeatures/index.src.html

@@ -659,8 +659,8 @@ <h2 id="detection">Feature Detection</h2>
   Both {{Window}}'s {{Window/isSecureContext}} and {{WorkerGlobalScope}}'s
   {{WorkerGlobalScope/isSecureContext}} attribute's getters return
   <code>true</code> if [[#settings-secure]] returns <code>Secure</code> when
-  executed on the <a>incumbent settings object</a>, and <code>false</code>
-  otherwise.
+  executed on the <a>relevant settings object</a> for the getter's <a>global
+  object</a>, and <code>false</code> otherwise.
 </section>
 
 <section>