CustomElements: adopt node

third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js

 function create_window_in_test(t, srcdoc) {
   let p = new Promise((resolve) => {
     let f = document.createElement('iframe');
     f.srcdoc = srcdoc ? srcdoc : '';
     f.onload = (event) => {
       let w = f.contentWindow;
       t.add_cleanup(() => f.remove());
       resolve(w);
     };
     document.body.appendChild(f);
@@ skipping 14 matching lines @@
   let exception;
   assert_throws(code, () => {
     try {
       func.call(this);
     } catch(e) {
       exception = e;
       throw e;
     }
   }, description);
   assert_true(exception instanceof global_context.DOMException, 'DOMException on the appropriate window');
-}
+}
+
+function assert_array_equals_callback_invocations(actual, expected, description) {
+  assert_equals(actual.length, expected.length);
+  for (let len=actual.length, i=0; i<len; ++i) {
+    let callback = expected[i][0];
+    assert_equals(actual[i][0], expected[i][0], callback + ' callback should be invoked');
+    assert_equals(actual[i][1], expected[i][1], callback + ' should be invoked on the element ' + expected[i][1]);
+    assert_array_equals(actual[i][2], expected[i][2], callback + ' should be invoked with the arguments ' + expected[i][2]);
+  }
+}