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('a-a', MyElement);
+
+  // createElement should synchronously call constructor.
+  let a = document.createElement('a-a');
+  assert_equals(constructors.length, 1);
+  assert_equals(a.ownerDocument, document);
+
+  let importDoc = document.currentScript.ownerDocument;
+
+  // TODO(kochi): crbug.com/640465 createElement returns wrong ownerDocument
+  // createElement should work in imported document.
+  let b = importDoc.createElement('a-a')
+  assert_equals(b.ownerDocument, importDoc);
+  assert_equals(constructors.length, 2);
+
+  // new MyElement() should synchronously call constructor.
+  let c = new MyElement();
+  assert_equals(c.ownerDocument, document);
+  assert_equals(constructors.length, e);
+
+  assert_array_equals(constructors, [a, b, c]);
+}, 'createElement() and new MyElement should work in imported document.');
+</script>