Add 'creator context security'

Secure Contexts relied on 'creator Document', which was removed in
whatwg#987. This patch caches the security status of that
Document for the new browsing context's security checks.

w3c/webappsec-secure-contexts#37

source

@@ -77215,13 +77215,29 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
   context</span>, then <em>that</em> is its <span>creator browsing context</span>. Otherwise, the
   <span>browsing context</span> has no <span>creator browsing context</span>.</p>
 
-  <p>If a <span>browsing context</span> <var>A</var> has a <span>creator browsing
-  context</span>, then the <dfn>creator origin</dfn>, <dfn>creator URL</dfn>, <dfn>creator base
-  URL</dfn>, and <dfn>creator referrer policy</dfn> are the <span>origin</span>, <span
-  data-x="concept-document-url">URL</span>, <span data-x="document base URL">base URL</span>, and
-  <span data-x="concept-document-referrer-policy">referrer policy</span>, respectively, of the
-  <code>Document</code> that was the <span>active document</span> of that <span>creator browsing
-  context</span> at the time <var>A</var> was created.</p>
+  <p>If a <span>browsing context</span> <var>context</var> has a <span>creator browsing
+  context</span> <var>creator</var>, it also has the following properties. In what follows, let
+  <var>creator document</var> be <var>creator</var>'s <span>active document</span> at the time
+  <var>context</var> is created:</p>
+
+  <dl>
+   <dt><dfn>creator origin</dfn></dt>
+   <dd><var>creator document</var>'s <span>origin</span></dd>
+
+   <dt><dfn>creator URL</dfn></dt>
+   <dd><var>creator document</var>'s <span data-x="concept-document-url">URL</span></dd>
+
+   <dt><dfn>creator base URL</dfn></dt>
+   <dd><var>creator document</var>'s <span data-x="document base URL">base URL</span></dd>
+
+   <dt><dfn>creator referrer policy</dfn></dt>
+   <dd><var>creator document</var>'s <span data-x="concept-document-referrer-policy">referrer
+   policy</span></dd>
+
+   <dt><dfn>creator context security</dfn></dt>
+   <dd>The result of executing <span>Is environment settings object a secure context?</span> on
+   <var>creator document</var>'s <span>relevant settings object</span></dd>
+  </dl>
 
   <div w-nodev>