Make attachShadow() use the global registry by default

This significantly improves the reuse of custom elements in different registries.

Tests: web-platform-tests/wpt#55248.

Fixes #1407.

Author: annevk

dom.bs

@@ -7583,7 +7583,8 @@ are:
 <p>The <dfn method for=Element><code>attachShadow(<var>init</var>)</code></dfn> method steps are:
 
 <ol>
- <li><p>Let <var>registry</var> be <a>this</a>'s <a for=Element>custom element registry</a>.
+ <li><p>Let <var>registry</var> be <a>this</a>'s <a>node document</a>'s
+ <a for=Document>custom element registry</a>.
 
  <li>
   <p>If <var>init</var>["{{ShadowRootInit/customElementRegistry}}"] is non-null: