gclient validate: allow None in deps_os values
gclient validate: allow None in deps_os values

Bug: 722206
Change-Id: I15024670e481c166ca34a2ec8dac93c739684121
Reviewed-on: https://chromium-review.googlesource.com/509537
Reviewed-by: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <84cab74a96a175cf90ed68b8abab420352923e11@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools