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

chrome/browser/chromeos/policy/device_local_account_policy_service.h

Index: chrome/browser/chromeos/policy/device_local_account_policy_service.h
diff --git a/chrome/browser/chromeos/policy/device_local_account_policy_service.h b/chrome/browser/chromeos/policy/device_local_account_policy_service.h
index de766394ead77925c9eb29a6438b3099024cc59b..68613e4ff296b9515fbf34bdd8ec01d4997004f9 100644
--- a/chrome/browser/chromeos/policy/device_local_account_policy_service.h
+++ b/chrome/browser/chromeos/policy/device_local_account_policy_service.h
@@ -25,6 +25,7 @@
 #include "components/policy/core/common/cloud/cloud_policy_store.h"
 #include "components/policy/core/common/cloud/component_cloud_policy_service.h"
 #include "components/policy/core/common/schema_registry.h"
+#include "components/user_manager/user_id.h"
 
 namespace base {
 class SequencedTaskRunner;
@@ -75,7 +76,7 @@ class DeviceLocalAccountPolicyBroker
   // For the difference between |account_id| and |user_id|, see the
   // documentation of DeviceLocalAccount.
   const std::string& account_id() const { return account_id_; }
-  const std::string& user_id() const { return user_id_; }
+  const user_manager::UserID& user_id() const { return user_id_; }
 
   scoped_refptr<chromeos::DeviceLocalAccountExternalPolicyLoader>
       extension_loader() const { return extension_loader_; }
@@ -123,7 +124,7 @@ class DeviceLocalAccountPolicyBroker
 
   AffiliatedInvalidationServiceProvider* const invalidation_service_provider_;
   const std::string account_id_;
-  const std::string user_id_;
+  const user_manager::UserID user_id_;
   const base::FilePath component_policy_cache_path_;
   SchemaRegistry schema_registry_;
   const scoped_ptr<DeviceLocalAccountPolicyStore> store_;
@@ -151,7 +152,7 @@ class DeviceLocalAccountPolicyService {
     virtual ~Observer() {}
 
     // Policy for the given |user_id| has changed.
-    virtual void OnPolicyUpdated(const std::string& user_id) = 0;
+    virtual void OnPolicyUpdated(const user_manager::UserID& user_id) = 0;
 
     // The list of accounts has been updated.
     virtual void OnDeviceLocalAccountsChanged() = 0;
@@ -178,17 +179,17 @@ class DeviceLocalAccountPolicyService {
 
   // Get the policy broker for a given |user_id|. Returns NULL if that |user_id|
   // does not belong to an existing device-local account.
-  DeviceLocalAccountPolicyBroker* GetBrokerForUser(const std::string& user_id);
+  DeviceLocalAccountPolicyBroker* GetBrokerForUser(const user_manager::UserID& user_id);
 
   // Indicates whether policy has been successfully fetched for the given
   // |user_id|.
-  bool IsPolicyAvailableForUser(const std::string& user_id);
+  bool IsPolicyAvailableForUser(const user_manager::UserID& user_id);
 
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
  private:
-  typedef std::map<std::string, DeviceLocalAccountPolicyBroker*>
+  typedef std::map<user_manager::UserID, DeviceLocalAccountPolicyBroker*>
       PolicyBrokerMap;
 
   // Returns |true| if the directory in which force-installed extensions are
@@ -229,7 +230,7 @@ class DeviceLocalAccountPolicyService {
   DeviceLocalAccountPolicyBroker* GetBrokerForStore(CloudPolicyStore* store);
 
   // Notifies the |observers_| that the policy for |user_id| has changed.
-  void NotifyPolicyUpdated(const std::string& user_id);
+  void NotifyPolicyUpdated(const user_manager::UserID& user_id);
 
   ObserverList<Observer, true> observers_;