Make assert_reports_js use assert_throws_js

This was originally left out of https://crrev.com/d255670 due to an
error when trying to use assert_throws_js. After investigation, it turns
out just to be that the function using assert_reports_js was not passing
a TypeError from the right global.

Bug: None
Change-Id: I8067949ddf6e9108bd862287cb49eef7aa788eff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2128243Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755070}

third_party/blink/web_tests/custom-elements/spec/construct.html

@@ -146,7 +146,7 @@ test_with_window((w) => {
   }
   w.customElements.define('a-a', A);
   let d = w.document.createElement('div');
-  assert_reports_js(window, TypeError, () => {
+  assert_reports_js(window, w.TypeError, () => {
     d.innerHTML = '<a-a>';
   }, 'Creating an element that is already constructed marker should report ' +
      'InvalidStateError');

third_party/blink/web_tests/custom-elements/spec/resources/custom-elements-helpers.js

@@ -97,8 +97,5 @@ function assert_reports_js(w, expected_error, func, description) {
     w.onerror = old_onerror;
   }
   assert_equals(errors.length, 1, 'only one error should have been reported');
-  assert_true(
-      typeof errors[0] === 'object' && errors[0] !== null,
-      'got something other than an error');
-  assert_equals(expected_error.name, errors[0].name);
+  assert_throws_js(expected_error, () => { throw errors[0]; });
 }