Add an option to not store stdout when running commands unless it's needed.

R=agable@chromium.org

Bug: 722686
Change-Id: I402c83767097d53e588ba3e8bca89291712a572f
Reviewed-on: https://chromium-review.googlesource.com/888584Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>