As a step of Context-associated MicrotaskQueue [1], this CL adds
a stub for v8::internal::MicrotaskQueue.

[1]: https://docs.google.com/document/d/1oNNhXF66dmwyj5S6Ri_g1vmBZltK8rWvZo1Tgqj64Z0/edit?usp=sharing

Tbr: bmeurer@chromium.org
Bug: v8:8124
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ie3d86ca49d46749660ec1316e4ae051528076e97
Reviewed-on: https://chromium-review.googlesource.com/1192803
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55624}