Make custom element definition trigger in-document upgrades

This was part of the previous conclusion in
WICG/webcomponents#419, but never got added
back to the spec:

- WICG/webcomponents@c4a829a
  removed all auto-upgrading
- WICG/webcomponents@70b9d8d
  only added it back for when you insert an element into a document, not
  for when you define an element.

Author: domenic

source

@@ -67655,7 +67655,7 @@ console.log(plasticButton2.getAttribute("is")); // will output "plastic-button"<
   <code>Window</code> object.</p>
 
   <pre class="idl">interface <dfn>CustomElementsRegistry</dfn> {
-  void <span data-x="dom-CustomElementsRegistry-define">define</span>(DOMString name, Function constructor, optional ElementRegistrationOptions options);
+  [<span>CEReactions</span>] void <span data-x="dom-CustomElementsRegistry-define">define</span>(DOMString name, Function constructor, optional ElementRegistrationOptions options);
   any <span data-x="dom-CustomElementsRegistry-get">get</span>(DOMString name);
   [NewObject] Promise&lt;void&gt; <span data-x="dom-CustomElementsRegistry-whenDefined">whenDefined</span>(DOMString name);
 };
@@ -67811,6 +67811,22 @@ dictionary <dfn>ElementRegistrationOptions</dfn> {
 
    <li><p>Add <var>definition</var> to this <code>CustomElementsRegistry</code>.</p></li>
 
+   <li><p>Let <var>document</var> be this <code>CustomElementsRegistry</code>'s <span
+   data-x="concept-relevant-global">relevant global object</span>'s <span
+   data-x="concept-document-window"><code>Document</code> object</span>.</p></li>
+
+   <li><p>Let <var>upgrade candidates</var> be all elements in <var>document</var> whose namespace
+   is the <span>HTML namespace</span> and whose local name is <var>localName</var>, in tree
+   order. Additionally, if <var>extends</var> is non-null, only include elements that have an
+   attribute named <code data-x="attr-is">is</code> whose value is <var>name</var>.</p></li>
+
+   <li><p>For each element <var>element</var> in <var>upgrade candidates</var>, <span>enqueue a
+   custom element upgrade reaction</span> given <var>element</var> and
+   <var>definition</var>.</p></li>
+
+   <!-- It is equivalent to just try to upgrade all elements in the document, and let "try to
+   upgrade" bail out, but this seems a bit more explicit. -->
+
    <li>
     <p>If this <code>CustomElementsRegistry</code>'s <span>when-defined promise map</span>
     contains an entry with key <var>name</var>:</p>