Use promise_rejects in portals-activate-no-browsing-context.html.

Change-Id: Iccdbbe823d3d4ed2554c2527d90da8b7692a2e4e
Reviewed-on: https://chromium-review.googlesource.com/c/1450282
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Auto-Submit: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628489}

third_party/blink/web_tests/external/wpt/portals/portals-activate-no-browsing-context.html

@@ -2,11 +2,8 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script>
-promise_test(async () => {
+promise_test(async t => {
   let activatePromise = document.createElement('portal').activate();
-  await activatePromise.then(() => assert_unreached(), e => {
-    assert_true(e instanceof DOMException);
-    assert_equals(e.name, 'InvalidStateError');
-  });
+  await promise_rejects(t, 'InvalidStateError', activatePromise);
 }, "A portal with nothing in it cannot be activated");
 </script>