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

chrome/browser/chromeos/policy/policy_cert_service_factory.h

 // Copyright 2013 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.
 
 #ifndef CHROME_BROWSER_CHROMEOS_POLICY_POLICY_CERT_SERVICE_FACTORY_H_
 #define CHROME_BROWSER_CHROMEOS_POLICY_POLICY_CERT_SERVICE_FACTORY_H_
 
-#include <string>
-
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
 template <typename T> struct DefaultSingletonTraits;
 
 class PrefRegistrySimple;
 class Profile;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace policy {
 
 class PolicyCertService;
 class PolicyCertVerifier;
 
 // Factory to create PolicyCertServices.
 class PolicyCertServiceFactory : public BrowserContextKeyedServiceFactory {
  public:
   // Returns an existing PolicyCertService for |profile|. See
   // CreateForProfile.
   static PolicyCertService* GetForProfile(Profile* profile);
 
   // Creates a new PolicyCertService and returns the associated
   // PolicyCertVerifier. Returns NULL if this service isn't allowed for
   // |profile|, i.e. if NetworkConfigurationUpdater doesn't exist.
   // This service is created separately for the original profile and the
   // incognito profile.
   // Note: NetworkConfigurationUpdater is currently only created for the primary
   // user's profile.
   static scoped_ptr<PolicyCertVerifier> CreateForProfile(Profile* profile);
 
   static PolicyCertServiceFactory* GetInstance();
 
   // Used to mark or clear |user_id| as having used certificates pushed by
   // policy before.
-  static void SetUsedPolicyCertificates(const std::string& user_id);
-  static void ClearUsedPolicyCertificates(const std::string& user_id);
-  static bool UsedPolicyCertificates(const std::string& user_id);
+  static void SetUsedPolicyCertificates(const user_manager::UserID& user_id);
+  static void ClearUsedPolicyCertificates(const user_manager::UserID& user_id);
+  static bool UsedPolicyCertificates(const user_manager::UserID& user_id);
 
   static void RegisterPrefs(PrefRegistrySimple* local_state);
 
  private:
   friend struct DefaultSingletonTraits<PolicyCertServiceFactory>;
 
   PolicyCertServiceFactory();
   ~PolicyCertServiceFactory() override;
 
   // BrowserContextKeyedServiceFactory:
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
   content::BrowserContext* GetBrowserContextToUse(
       content::BrowserContext* context) const override;
   void RegisterProfilePrefs(
       user_prefs::PrefRegistrySyncable* registry) override;
   bool ServiceIsNULLWhileTesting() const override;
 
   DISALLOW_COPY_AND_ASSIGN(PolicyCertServiceFactory);
 };
 
 }  // namespace policy
 
 #endif  // CHROME_BROWSER_CHROMEOS_POLICY_POLICY_CERT_SERVICE_FACTORY_H_