Make custom elements work in HTML imports

third_party/WebKit/LayoutTests/custom-elements/imports/resources/create-element.html

+<script>
+'use strict';
+let constructors = [];
+test(() => {
+  assert_equals(constructors.length, 0);
+
+  class MyElement extends HTMLElement {
+    constructor() {
+      super();
+      constructors.push(this);
+    }
+  }
+
+  customElements.define('my-element', MyElement);
+
+  // createElement should synchronously call constructor.
+  let el = document.createElement('my-element');
+  assert_equals(constructors.length, 1);
+
+  let importDoc = document.currentScript.ownerDocument;
+  let sandbox = importDoc.querySelector('#sandbox');
+
+  // createElement should work in imported document.
+  let el2 = importDoc.createElement('my-element')
+  assert_equals(constructors.length, 2);
+
+  // new MyElement() should synchronously call constructor.
+  let el3 = new MyElement();
+  assert_equals(constructors.length, 3);
+
+  assert_array_equals(constructors, [el, el2, el3]);
+}, 'createElement() should work in imported document.');
+</script>