[step_runner] Specify close_fds to avoid leaking non-std handles to steps.
[step_runner] Specify close_fds to avoid leaking non-std handles to steps.

R=5de1a387cc854ff3d04fab3d66d4e77a58511771@chromium.org, martiniss@chromium.org, tandrii@chromium.org

Bug: 929965
Change-Id: I5aaeaadfc0578565591e50828c9b2bc9d0dde701
Reviewed-on: https://chromium-review.googlesource.com/c/1460319
Reviewed-by: Jao-ke Chin-Lee <5de1a387cc854ff3d04fab3d66d4e77a58511771@chromium.org>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/recipes-py,luci/recipes-py