Make initialize() upgrade and fix upgrade()

CustomElementRegistry's upgrade() wasn't adjusted for scoped registries and we decided that initialize() should upgrade as well (but not into shadow trees as upgrade() does).

The initialize() design here favors a single tree traversal for setting the registry and then upgrading the element.

Tests:
- web-platform-tests/wpt#56034
- web-platform-tests/wpt#56132

Fixes #11858 and fixes #11908.

Author: annevk

source

@@ -77277,12 +77277,19 @@ fetch(articleURL)
   are:</p>
 
   <ol>
-   <li><p>Let <var>candidates</var> be a <span>list</span> of all of <var>root</var>'s
-   <span>shadow-including inclusive descendant</span> elements, in <span>shadow-including tree
-   order</span>.</p></li>
+   <li>
+    <p>For each <span>shadow-including inclusive descendant</span> <var>candidate</var> of
+    <var>root</var>, in <span>shadow-including tree order</span>:</p>
+
+    <ol>
+     <li><p>If <var>candidate</var> is not an <code>Element</code> node, then
+     <span>continue</span>.</p></li>
 
-   <li><p><span data-x="list iterate">For each</span> <var>candidate</var> of <var>candidates</var>,
-   <span data-x="concept-try-upgrade">try to upgrade</span> <var>candidate</var>.</p></li>
+     <li><p>If <var>candidate</var>'s <span data-x="element-custom-element-registry">custom element
+     registry</span> is not <span>this</span>, then <span>continue</span>.</p></li>
+
+     <li><p><span data-x="concept-try-upgrade">Try to upgrade</span> <var>candidate</var>.</p></li>
+    </ol>
   </ol>
   </div>
 
@@ -77330,18 +77337,34 @@ console.assert(el instanceof SpiderMan);    // upgraded!</code></pre>
    registry</span> to <span>this</span>.</p></li>
 
    <li>
-    <p>For each <span>inclusive descendant</span> <var>inclusiveDescendant</var> of <var>root</var>:
-    if <var>inclusiveDescendant</var> is an <code>Element</code> node whose <span
-    data-x="element-custom-element-registry">custom element registry</span> is null:</p>
+    <p>For each <span>inclusive descendant</span> <var>inclusiveDescendant</var> of <var>root</var>,
+    in <span>tree order</span>:</p>
 
     <ol>
-     <li><p>Set <var>inclusiveDescendant</var>'s <span
-     data-x="element-custom-element-registry">custom element registry</span> to
-     <span>this</span>.</p></li>
+     <li><p>If <var>inclusiveDescendant</var> is not an <code>Element</code> node, then
+     <span>continue</span>.</p></li>
+
+     <li>
+      <p>If <var>inclusiveDescendant</var>'s <span data-x="element-custom-element-registry">custom
+      element registry</span> is null:</p>
+
+      <ol>
+       <li><p>Set <var>inclusiveDescendant</var>'s <span
+       data-x="element-custom-element-registry">custom element registry</span> to
+       <span>this</span>.</p></li>
+
+       <li><p>If <span>this</span>'s <span>is scoped</span> is true, then <span data-x="set
+       append">append</span> <var>inclusiveDescendant</var>'s <span>node document</span> to
+       <span>this</span>'s <span>scoped document set</span>.</p></li>
+      </ol>
+     </li>
+
+     <li><p>If <var>inclusiveDescendant</var>'s <span
+     data-x="element-custom-element-registry">custom element registry</span> is not
+     <span>this</span>, then <span>continue</span>.</p></li>
 
-     <li><p>If <span>this</span>'s <span>is scoped</span> is true, then <span data-x="set
-     append">append</span> <var>inclusiveDescendant</var>'s <span>node document</span> to
-     <span>this</span>'s <span>scoped document set</span>.</p></li>
+     <li><p><span data-x="concept-try-upgrade">Try to upgrade</span>
+     <var>inclusiveDescendant</var>.</p></li>
     </ol>
    </li>
   </ol>