[WPT] Microtasks checkpoint around event handlers

Failures on Chromium:

- crbug/1114034
    - checkpoint-after-window-onerror-module.html
    - checkpoint-after-workerglobalscope-onerror-module.html
- crbug/1114028
    - checkpoint-after-workerglobalscope-onerror.html

Bug: 1096455c, 1114028, 1114034
Change-Id: If0b6ed9110decccd01cae5120c96390894df4047
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341267


Reviewed-by: Domenic Denicola <domenic@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796714}