Add worklets to JavaScript agent clusters

Fixes w3c/css-houdini-drafts#380.

Tests: web-platform-tests/wpt@2c89bbe#diff-b84a61f825cf43affca063e7c09e4ae8.

source

@@ -4174,6 +4174,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
 
     <ul class="brief">
      <li><dfn data-x-href="https://drafts.css-houdini.org/worklets/#workletglobalscope"><code>WorkletGlobalScope</code></dfn></li>
+     <li>A <code>WorkletGlobalScope</code>'s <dfn data-x="concept-worklet-owner-document" data-x-href="https://drafts.css-houdini.org/worklets/#workletglobalscope-owner-document">owner document</dfn></li>
     </ul>
    </dd>
 
@@ -88662,10 +88663,11 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
   <p class="note">"Item" is used above as an <span>owner set</span> can contain
   <code>Document</code> objects.</p>
 
-  <p class="&#x0058;&#x0058;&#x0058;">A <span>worklet agent</span> &hellip; currently worklets have
-  no clearly defined owner, see: <a
-  href="https://github.com/w3c/css-houdini-drafts/issues/224">w3c/css-houdini-drafts issue
-  #224</a>.</p>
+  <p>A <span>similar-origin window agent</span> <span>can share memory with</span> any
+  <span>worklet agent</span> whose single <span data-x="JavaScript realm">realm</span>'s
+  <span data-x="concept-realm-global">global object</span>'s
+  <span data-x="concept-worklet-owner-document">owner document</span>'s
+  <span data-x="concept-relevant-realm">relevant Realm</span> belongs to <var>agent</var>.</p>
 
   <p>In addition, any <span>agent</span> <var>A</var> <span>can share memory with</span>:</p>
 
@@ -88695,6 +88697,8 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
 
     <li>A <code>Window</code> object and a <span>same origin-domain</span> <code>Window</code>
     object that opened it.</li>
+
+    <li>A <code>Window</code> object and a worklet that it created.</li>
    </ul>
 
    <p>The following pairs of global objects are <em>not</em> within the same <span>agent