Revert "gclient validate: allow None in deps_os values"

This reverts commit 568e591944.

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 <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

gclient_eval.py

@@ -40,9 +40,7 @@ _GCLIENT_SCHEMA = schema.Schema({
     schema.Optional('deps'): {schema.Optional(basestring): basestring},
 
     # Similar to 'deps' (see above) - also keyed by OS (e.g. 'linux').
-    schema.Optional('deps_os'): {
-        basestring: {basestring: schema.Or(basestring, None)}
-    },
+    schema.Optional('deps_os'): {basestring: {basestring: basestring}},
 
     # Hooks executed after gclient sync (unless suppressed), or explicitly
     # on gclient hooks. See _GCLIENT_HOOKS_SCHEMA for details.