Add self.crossOriginIsolated

Closes #4872. (I'm not convinced we need to add crossOriginOpener and crossOriginEmbedder for now.)

Author: annevk

source

@@ -93299,6 +93299,7 @@ interface mixin <dfn>DocumentAndElementEventHandlers</dfn> {
 
 interface mixin <dfn>WindowOrWorkerGlobalScope</dfn> {
   [Replaceable] readonly attribute USVString <span data-x="dom-origin">origin</span>;
+  readonly attribute boolean <span data-x="dom-crossOriginIsolated">crossOriginIsolated</span>;
 
   // base64 utility methods
   DOMString <span data-x="dom-btoa">btoa</span>(DOMString data);
@@ -93322,8 +93323,17 @@ interface mixin <dfn>WindowOrWorkerGlobalScope</dfn> {
 
   <dl class="domintro">
    <dt><var>origin</var> = self . <code subdfn data-x="dom-origin">origin</code></dt>
-
    <dd><p>Returns the global object's <span>origin</span>, serialized as string.</p></dd>
+
+   <dt><var>origin</var> = self . <code subdfn data-x="dom-crossOriginIsolated">crossOriginIsolated</code></dt>
+   <dd>
+    <p>Returns the <span>surrounding agent</span>'s <span>agent cluster</span>'s
+    <span>cross-origin isolated</span>.</p>
+
+    <p>(This value depends on the <code data-x="">Cross-Origin-Opener-Policy</code> and <code
+    data-x="">Cross-Origin-Embedder-Policy</code> HTTP response headers and determines whether
+    <code>SharedArrayBuffer</code> can be used with <code data-x="">postMessage()</code> APIs.)</p>
+   </dd>
   </dl>
 
   <div class="example">
@@ -93348,6 +93358,10 @@ document.body.appendChild(frame)</code></pre>
   data-x="concept-settings-object-origin">origin</span>, <span data-x="serialization of an
   origin">serialized</span>.</p>
 
+  <p>The <dfn data-x="dom-crossOriginIsolated"><code>crossOriginIsolated</code></dfn> attribute's
+  getter must return the <span>surrounding agent</span>'s <span>agent cluster</span>'s
+  <span>cross-origin isolated</span>.</p>
+
 
   <h3 id="atob">Base64 utility methods</h3>