We should use UserID object to identify users instead of username.

chrome/browser/chromeos/policy/user_cloud_policy_manager_chromeos.h

Index: chrome/browser/chromeos/policy/user_cloud_policy_manager_chromeos.h
diff --git a/chrome/browser/chromeos/policy/user_cloud_policy_manager_chromeos.h b/chrome/browser/chromeos/policy/user_cloud_policy_manager_chromeos.h
index a7df866b14e2d15bd5f7810784376992fb5cab4d..18b2dfa36f60d7bea1f37509e9cd41f72af331c4 100644
--- a/chrome/browser/chromeos/policy/user_cloud_policy_manager_chromeos.h
+++ b/chrome/browser/chromeos/policy/user_cloud_policy_manager_chromeos.h
@@ -19,6 +19,7 @@
 #include "components/policy/core/common/cloud/cloud_policy_constants.h"
 #include "components/policy/core/common/cloud/cloud_policy_manager.h"
 #include "components/policy/core/common/cloud/cloud_policy_service.h"
+#include "components/user_manager/user_id.h"
 
 class GoogleServiceAuthError;
 class PrefService;
@@ -31,6 +32,10 @@ namespace net {
 class URLRequestContextGetter;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace policy {
 
 class CloudExternalDataManager;
@@ -87,7 +92,7 @@ class UserCloudPolicyManagerChromeOS : public CloudPolicyManager,
 
   // Indicates a wildcard login check should be performed once an access token
   // is available.
-  void EnableWildcardLoginCheck(const std::string& username);
+  void EnableWildcardLoginCheck(const user_manager::UserID& user_id);
 
   // ConfigurationPolicyProvider:
   void Shutdown() override;
@@ -142,7 +147,7 @@ class UserCloudPolicyManagerChromeOS : public CloudPolicyManager,
   scoped_ptr<CloudExternalDataManager> external_data_manager_;
 
   // Username for the wildcard login check if applicable, empty otherwise.
-  std::string wildcard_username_;
+  user_manager::UserID wildcard_user_id_;
 
   // Path where policy for components will be cached.
   base::FilePath component_policy_cache_path_;