This seperates OS-specific code of the Command class into a Posix and a
WindowsCommand to simplify scattered OS checks.

This also removes some temporary mac debug output that's obsolete after
https://crbug.com/v8/6927 got resolved.

TBR=sergiyb@chromium.org

Bug: v8:6917
Change-Id: Iaab3b527ce556dfba797a164ae58e8dd358eb56f
Reviewed-on: https://chromium-review.googlesource.com/847000Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50346}