Also make autoninja.py a vpython script to use reliable version of
psutil.

Note: this change also makes autoninja always make a decision about
-j; there's no longer a default where it lets ninja pick. The code is
simpler this way and I think it's better because it lets developers
always see which -j is in effect when using autoninja (and that's its
exact purpose, if you wanted default you shouldn't have used autoninja).

R=dpranke@chromium.org

Bug: 976265
Change-Id: Ic9d12469729e4bf58da1ec1bd70437329519fc46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1663904Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>