gclient: expose target_os
gclient: expose target_os

Bug: 570091
Change-Id: I7513f3360662975ef6ef1d9019726dcef730a586
Reviewed-on: https://chromium-review.googlesource.com/681700
Commit-Queue: Paweł Hajdan Jr. <84cab74a96a175cf90ed68b8abab420352923e11@chromium.org>
Reviewed-by: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools