MicrotaskQueue didn't free its buffer on GC phase if it's empty.

Bug: v8:8124
Change-Id: Icdd6a67873cab164dcf67ed1caf5cca55e3f7954
Reviewed-on: https://chromium-review.googlesource.com/c/1351856Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57891}