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

chrome/browser/chromeos/policy/device_local_account_policy_provider.h

Index: chrome/browser/chromeos/policy/device_local_account_policy_provider.h
diff --git a/chrome/browser/chromeos/policy/device_local_account_policy_provider.h b/chrome/browser/chromeos/policy/device_local_account_policy_provider.h
index 3ff0f56a8317b49ce28a9637e4c0249cf7f87160..82358d2afc52dfe2be8e230baa1e7a6892f47103 100644
--- a/chrome/browser/chromeos/policy/device_local_account_policy_provider.h
+++ b/chrome/browser/chromeos/policy/device_local_account_policy_provider.h
@@ -15,6 +15,7 @@
 #include "chrome/browser/chromeos/policy/device_local_account_external_data_manager.h"
 #include "chrome/browser/chromeos/policy/device_local_account_policy_service.h"
 #include "components/policy/core/common/configuration_policy_provider.h"
+#include "components/user_manager/user_id.h"
 
 namespace policy {
 
@@ -31,7 +32,7 @@ class DeviceLocalAccountPolicyProvider
       public DeviceLocalAccountPolicyService::Observer {
  public:
   DeviceLocalAccountPolicyProvider(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       DeviceLocalAccountPolicyService* service,
       scoped_ptr<PolicyMap> chrome_policy_overrides);
   ~DeviceLocalAccountPolicyProvider() override;
@@ -40,7 +41,7 @@ class DeviceLocalAccountPolicyProvider
   // NULL if |user_id| is not a device-local account or user policy isn't
   // applicable for user_id's user type.
   static scoped_ptr<DeviceLocalAccountPolicyProvider> Create(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       DeviceLocalAccountPolicyService* service);
 
   // ConfigurationPolicyProvider:
@@ -48,7 +49,7 @@ class DeviceLocalAccountPolicyProvider
   void RefreshPolicies() override;
 
   // DeviceLocalAccountPolicyService::Observer:
-  void OnPolicyUpdated(const std::string& user_id) override;
+  void OnPolicyUpdated(const user_manager::UserID& user_id) override;
   void OnDeviceLocalAccountsChanged() override;
 
  private:
@@ -63,7 +64,7 @@ class DeviceLocalAccountPolicyProvider
   // policy from the broker if available or keeping the current policy.
   void UpdateFromBroker();
 
-  const std::string user_id_;
+  const user_manager::UserID user_id_;
   scoped_refptr<DeviceLocalAccountExternalDataManager> external_data_manager_;
 
   DeviceLocalAccountPolicyService* service_;