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

This reverts commit 568e5919442f1dd3a2b160dbe998d1f1489a09c9.

Reason for revert: maybe caused outage

Original change's description:
> 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 <dpranke@chromium.org>
> Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
> 

TBR=phajdan.jr@chromium.org,dpranke@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
Bug: 722206

Change-Id: Ia0f3825d204e65575020d108873dee82da47a1e6
Reviewed-on: https://chromium-review.googlesource.com/512345
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools