Adjust custom element registry initialization for declarative shadow trees

In whatwg/dom#1409 this was changed for attachShadow(), but we forgot to change the HTML parser at the same time.

Tests: web-platform-tests/wpt#56134.

Fixes #11947.

Author: annevk

source

@@ -140339,12 +140339,10 @@ document.body.appendChild(text);
         <p>Otherwise:</p>
 
         <ol>
-         <li><p>Let <var>registry</var> be <var>declarativeShadowHostElement</var>'s <span
-         data-x="element-custom-element-registry">custom element registry</span>.</p></li>
-
-         <li><p>If <var>templateStartTag</var> has a <code
+         <li><p>Let <var>registry</var> be null if <var>templateStartTag</var> has a <code
          data-x="attr-template-shadowrootcustomelementregistry">shadowrootcustomelementregistry</code>
-         attribute, then set <var>registry</var> to null.</p></li>
+         attribute; otherwise <var>declarativeShadowHostElement</var>'s <span>node document</span>'s
+         <span data-x="element-custom-element-registry">custom element registry</span>.</p></li>
 
          <li>
           <p><span data-x="concept-attach-a-shadow-root">Attach a shadow root</span> with