Login screen and Guest mode should use device owner preference

chrome/browser/policy/proto/chromeos/chrome_device_policy.proto

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package enterprise_management;
 
@@ skipping 218 matching lines @@
   // that are equivalent to one of the timezones in "timezone_settings.cc" are
   // valid. In case of an invalid value, the setting is still activated with a
   // fallback timezone (currently "GMT"). In case of an empty string or if no
   // value is provided, the timezone device setting is inactive. In that case,
   // the currently active timezone will remain in use however users can change
   // the timezone and the change is persistent. Thus a change by one user
   // affects the login-screen and all other users.
   optional string timezone = 1;
 }
 
+message SystemUse24HourClockProto {
+  // Specifies an owner-determined clock format that applies to the login
+  // screen and all users.
+  optional bool use_24hour_clock = 1;
+}
+
 // Parameters for Kiosk App device-local accounts.
 message KioskAppInfoProto {
   // Indicates the Kiosk App for the corresponding device-local account. The
   // string value should be a valid 32-character Chrome App identifier and
   // specifies the Kiosk App to download and run.
   optional string app_id = 1;
 
   // Optional extension update URL to download the Kiosk App package from. If
   // not specified, the app will be downloaded from the standard Chrome Web
   // Store update URL.
@@ skipping 283 matching lines @@
   optional SystemTimezoneProto system_timezone = 20;
   optional DeviceLocalAccountsProto device_local_accounts = 21;
   optional AllowRedeemChromeOsRegistrationOffersProto allow_redeem_offers = 22;
   optional StartUpFlagsProto start_up_flags = 23;
   optional UptimeLimitProto uptime_limit = 24;
   optional VariationsParameterProto variations_parameter = 25;
   optional AttestationSettingsProto attestation_settings = 26;
   optional AccessibilitySettingsProto accessibility_settings = 27;
   optional SupervisedUsersSettingsProto supervised_users_settings = 28;
   optional LoginScreenPowerManagementProto login_screen_power_management = 29;
+  optional SystemUse24HourClockProto use_24hour_clock = 30;
 }