Make custom elements work in HTML imports

third_party/WebKit/LayoutTests/custom-elements/imports/resources/parser-customize.html

+<parser-customize></parser-customize>
+<script>
+test(() => {
+  assert_equals(constructors.length, 0);
+
+  class ParserCustomize extends HTMLElement {
+    constructor() {
+      super();
+      constructors.push(document.currentScript.ownerDocument.title || 'Unknown');
+    }
+  }
+  customElements.define('parser-customize', ParserCustomize);
+
+  assert_equals(constructors.length, 1);
+}, 'Custom Element definition should upgrade parsed elements in import.');
+</script>
+
+<parser-customize></parser-customize>
+
+<script>
+test(() => {
+  assert_equals(constructors.length, 2);
+}, 'Custom Element definition should make parser customize elements.');
+</script>