Correctly find upgrade candidates during custom element definition

The previous imprecise language implied that shadow trees should be excluded. In fact, they should be included. Fixes #1263.

source

@@ -3143,12 +3143,11 @@ a.setAttribute('href', 'http://example.com/'); // change the content attribute d
      <li><dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#dom-node-textcontent"><code>textContent</code></dfn> attribute</li>
 
      <li>The <dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#concept-tree">tree</dfn> and <dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#concept-shadow-tree">shadow tree</dfn> concepts</li>
-     <li>The <dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#concept-tree-order">tree order</dfn> concept</li>
+     <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-tree-order">tree order</dfn> and <dfn data-x-href="https://dom.spec.whatwg.org/#concept-shadow-including-tree-order">shadow-including tree order</dfn> concepts</li>
      <li>The <dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#concept-tree-root">root</dfn> concept</li>
-     <li>The <dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#concept-tree-inclusive-ancestor">inclusive ancestor</dfn> concept</li>
+     <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-tree-inclusive-ancestor">inclusive ancestor</dfn> and <dfn data-x-href="https://dom.spec.whatwg.org/#concept-shadow-including-descendant">shadow-including descendant</dfn> concepts</li>
      <li>The <dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#document-element">document element</dfn> concept</li>
-     <li>The <dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#in-a-document">in a document</dfn> concept</li>
-     <li>The <dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#in-a-shadow-including-document">in a shadow-including document</dfn> concept</li>
+     <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#in-a-document">in a document</dfn> and <dfn data-x-href="https://dom.spec.whatwg.org/#in-a-shadow-including-document">in a shadow-including document</dfn> concepts</li>
      <li>The <dfn data-noexport="" data-x="concept-slot" data-x-href="https://dom.spec.whatwg.org/#concept-slot">slot</dfn> concept, and its <dfn data-noexport="" data-x="slot-name" data-x-href="https://dom.spec.whatwg.org/#slot-name">name</dfn> and <dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#slot-assigned-nodes">assigned nodes</dfn></li>
      <li>The <dfn data-noexport="" data-x="finding flattened slotables" data-x-href="https://dom.spec.whatwg.org/#find-flattened-slotables">find flattened slotables</dfn> algorithm</li>
      <li>The <dfn data-noexport="" data-x-href="https://dom.spec.whatwg.org/#concept-node-pre-insert">pre-insert</dfn>, <dfn data-noexport="" data-x="concept-node-insert" data-x-href="https://dom.spec.whatwg.org/#concept-node-insert">insert</dfn>, <dfn data-noexport="" data-x="concept-node-append" data-x-href="https://dom.spec.whatwg.org/#concept-node-append">append</dfn>, <dfn data-noexport="" data-x="concept-node-replace" data-x-href="https://dom.spec.whatwg.org/#concept-node-replace">replace</dfn>, <dfn data-noexport="" data-x="concept-node-remove" data-x-href="https://dom.spec.whatwg.org/#concept-node-remove">remove</dfn>, and <dfn data-noexport="" data-x="concept-node-adopt" data-x-href="https://dom.spec.whatwg.org/#concept-node-adopt">adopt</dfn> algorithms for nodes</li>
@@ -66291,10 +66290,12 @@ dictionary <dfn>ElementDefinitionOptions</dfn> {
    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>Let <var>upgrade candidates</var> be all elements that are <span data-x="shadow-including
+   descendant">shadow-including descendants</span> of <var>document</var>, whose namespace
+   is the <span>HTML namespace</span> and whose local name is <var>localName</var>, in
+   <span>shadow-including tree order</span>. 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