Make custom elements work in HTML imports

third_party/WebKit/LayoutTests/custom-elements/imports/resources/inner-html.html

+<div id="sandbox"></div>
+<script>
+'use strict';
+
+let constructors = [];
+
+test(() => {
+  assert_equals(constructors.length, 0);
+
+  customElements.define('a-a', class extends HTMLElement {
+    constructor() {
+      super();
+      constructors.push(this);
+    }
+  });
+
+  let importDoc = document.currentScript.ownerDocument;
+  let sandbox = importDoc.querySelector('#sandbox');
+
+  // createElement should synchronously customize.
+  sandbox.innerHTML = '<a-a id="x"></a-a>';
+
+  assert_equals(constructors.length, 1);
+  assert_equals(constructors[0].id, 'x');
+}, 'innerHTML with Custom Elements should work in imported document.');
+</script>