Factor out RunSingleMicrotask from RunMicrotasks builtin

This CL moves RunSingleMicrotask with no logic change as a non-essential
part of the following CL, in order to reduce the following CL size.

Change-Id: I93e4aa860710c169c0474ee28ada083139d2b607
Reviewed-on: https://chromium-review.googlesource.com/c/1328552
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57412}