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

chrome/browser/chromeos/policy/policy_cert_service.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_H_
 #define CHROME_BROWSER_CHROMEOS_POLICY_POLICY_CERT_SERVICE_H_
 
 #include <string>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/chromeos/policy/user_network_configuration_updater.h"
 #include "components/keyed_service/core/keyed_service.h"
+#include "components/user_manager/user_id.h"
 
 namespace user_manager {
 class UserManager;
 }
 
 namespace net {
 class X509Certificate;
 typedef std::vector<scoped_refptr<X509Certificate> > CertificateList;
 }
 
 namespace policy {
 
 class PolicyCertVerifier;
 
 // This service is the counterpart of PolicyCertVerifier on the UI thread. It's
 // responsible for pushing the current list of trust anchors to the CertVerifier
 // and marking the profile's prefs if any of the trust anchors was used.
 // Except for unit tests, PolicyCertVerifier should only be created through this
 // class.
 class PolicyCertService
     : public KeyedService,
       public UserNetworkConfigurationUpdater::WebTrustedCertsObserver {
  public:
-  PolicyCertService(const std::string& user_id,
+  PolicyCertService(const user_manager::UserID& user_id,
                     UserNetworkConfigurationUpdater* net_conf_updater,
                     user_manager::UserManager* user_manager);
   ~PolicyCertService() override;
 
   // Creates an associated PolicyCertVerifier. The returned object must only be
   // used on the IO thread and must outlive this object.
   scoped_ptr<PolicyCertVerifier> CreatePolicyCertVerifier();
 
   // Returns true if the profile that owns this service has used certificates
   // installed via policy to establish a secure connection before. This means
   // that it may have cached content from an untrusted source.
   bool UsedPolicyCertificates() const;
 
   bool has_policy_certificates() const { return has_trust_anchors_; }
 
   // UserNetworkConfigurationUpdater::WebTrustedCertsObserver:
   void OnTrustAnchorsChanged(
       const net::CertificateList& trust_anchors) override;
 
   // KeyedService:
   void Shutdown() override;
 
   static scoped_ptr<PolicyCertService> CreateForTesting(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       PolicyCertVerifier* verifier,
       user_manager::UserManager* user_manager);
 
  private:
-  PolicyCertService(const std::string& user_id,
+  PolicyCertService(const user_manager::UserID& user_id,
                     PolicyCertVerifier* verifier,
                     user_manager::UserManager* user_manager);
 
   PolicyCertVerifier* cert_verifier_;
-  std::string user_id_;
+  user_manager::UserID user_id_;
   UserNetworkConfigurationUpdater* net_conf_updater_;
   user_manager::UserManager* user_manager_;
   bool has_trust_anchors_;
 
   // Weak pointers to handle callbacks from PolicyCertVerifier on the IO thread.
   // The factory and the created WeakPtrs must only be used on the UI thread.
   base::WeakPtrFactory<PolicyCertService> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PolicyCertService);
 };
 
 }  // namespace policy
 
 #endif  // CHROME_BROWSER_CHROMEOS_POLICY_POLICY_CERT_SERVICE_H_