Login screen and Guest mode should use device owner preference

chrome/test/data/policy/policy_test_cases.json

 {
   "-- Template --": {
     "intro": "Top-level entries map a policy name to its test parameters, described below.",
 
     "os": ["List of operating systems that support this policy. Valid values:", "win", "linux", "mac", "chromeos", "Defaults to empty if not specified."],
     "official_only": "Whether this policy exists in official builds only. Defaults to |false| if not specified.",
     "can_be_recommended": "Whether a recommended value may be set for the policy. Defaults to |false| if not specified.",
     "test_policy": "A policy dictionary that should make the preferences affected by this policy become policy-controlled. Usually just sets the current policy. Defaults to an empty dictionary if not specified.",
     "note": "If the policy affects any preferences, the following array should be specified with one entry per such preference.",
     "pref_mappings": [
@@ skipping 1974 matching lines @@
 
   "DeviceUpdateAllowedConnectionTypes": {
   },
 
   "ReportDeviceLocation": {
   },
 
   "SystemTimezone": {
   },
 
+  "SystemUse24HourClock": {
+  },
+
   "DeviceLocalAccounts": {
   },
 
   "DeviceLocalAccountAutoLoginId": {
   },
 
   "DeviceLocalAccountAutoLoginDelay": {
   },
 
   "DeviceLocalAccountAutoLoginBailoutEnabled": {
@@ skipping 54 matching lines @@
 
   "GCFUserDataDir": {
   },
 
   "AdditionalLaunchParameters": {
   },
 
   "SuppressChromeFrameTurndownPrompt": {
   }
 }