Fix VirtualMachinesAllowed policy parsing for UI

Fix inconsistency in the code that parses this policy for the policy map
(which is used for the chrome://policy page). The code shouldn't pretend
to have the policy set (with the default value) when the enclosing proto
message is present but with the actual policy proto field unset.

Note that the fixed issue should have been a purely UI glitch - the
actual policy application wasn't affected.

BUG=chromium:800120
TEST=none

Change-Id: I32d55dc6bfe64ec8b1cac7c090028f805c91560a
Reviewed-on: https://chromium-review.googlesource.com/986096Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549849}

chrome/browser/chromeos/policy/device_policy_decoder_chromeos.cc

@@ -1051,11 +1051,13 @@ void DecodeGenericPolicies(const em::ChromeDeviceSettingsProto& policy,
   if (policy.has_virtual_machines_allowed()) {
     const em::VirtualMachinesAllowedProto& container(
         policy.virtual_machines_allowed());
-    policies->Set(
-        key::kVirtualMachinesAllowed, POLICY_LEVEL_MANDATORY,
-        POLICY_SCOPE_MACHINE, POLICY_SOURCE_CLOUD,
-        std::make_unique<base::Value>(container.virtual_machines_allowed()),
-        nullptr);
+    if (container.has_virtual_machines_allowed()) {
+      policies->Set(
+          key::kVirtualMachinesAllowed, POLICY_LEVEL_MANDATORY,
+          POLICY_SCOPE_MACHINE, POLICY_SOURCE_CLOUD,
+          std::make_unique<base::Value>(container.virtual_machines_allowed()),
+          nullptr);
+    }
   }
 
   if (policy.has_device_machine_password_change_rate()) {