Update links to unhandled promise rejection spec

The changes were merged into the WHATWG HTML Standard in
whatwg/html#224.

Change-Id: I21ca4eebdda22d4138ea844ccd6277b786bd5f36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2951212
Commit-Queue: Timothy Gu <timothygu@chromium.org>
Auto-Submit: Timothy Gu <timothygu@chromium.org>
Reviewed-by: Domenic Denicola <domenic@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#891344}

third_party/blink/renderer/core/events/promise_rejection_event.idl

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// https://github.com/domenic/unhandled-rejections-browser-spec
+// https://html.spec.whatwg.org/C/#promiserejectionevent
 
 [
     Exposed=(Window,Worker)

third_party/blink/renderer/core/events/promise_rejection_event_init.idl

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// https://github.com/domenic/unhandled-rejections-browser-spec
+// https://html.spec.whatwg.org/C/#promiserejectioneventinit
 
 dictionary PromiseRejectionEventInit : EventInit {
     required Promise<any> promise;