This is a reland of 516d9068

Original change's description:
> Expose the number of microtasks from RunMicrotasks
>
> This CL adds the number of processed microtasks to the tracing marker
> of RunMicrotasks, plus let RunMicrotasks return the number.
>
> Bug: v8:7804, v8:8124
> Change-Id: Ie584e22964121fbda3a822379d760e7518fc54a7
> Reviewed-on: https://chromium-review.googlesource.com/c/1425277
> Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
> Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#59017}

Bug: v8:7804, v8:8124
Change-Id: I4a57ba3e23973f6b46414c4502244091c42cf532
Reviewed-on: https://chromium-review.googlesource.com/c/1430399Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59148}