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

chrome/browser/chromeos/attestation/platform_verification_flow.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_ATTESTATION_PLATFORM_VERIFICATION_FLOW_H_
 #define CHROME_BROWSER_CHROMEOS_ATTESTATION_PLATFORM_VERIFICATION_FLOW_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
 #include "base/timer/timer.h"
 #include "url/gurl.h"
 
 namespace content {
 class WebContents;
 }
 
 namespace cryptohome {
 class AsyncMethodCaller;
 }
 
 namespace user_manager {
 class User;
+class UserID;
 }
 
 namespace chromeos {
 
 class CryptohomeClient;
 
 namespace attestation {
 
 class AttestationFlow;
 class PlatformVerificationFlowTest;
@@ skipping 124 matching lines @@
   // has been prepared on this device.
   void OnAttestationPrepared(const ChallengeContext& context,
                              bool attestation_prepared);
 
   // Initiates the flow to get a platform key certificate.  The arguments to
   // ChallengePlatformKey are in |context|.  |user_id| identifies the user for
   // which to get a certificate.  If |force_new_key| is true then any existing
   // key for the same user and service will be ignored and a new key will be
   // generated and certified.
   void GetCertificate(const ChallengeContext& context,
-                      const std::string& user_id,
+                      const user_manager::UserID& user_id,
                       bool force_new_key);
 
   // A callback called when an attestation certificate request operation
   // completes.  The arguments to ChallengePlatformKey are in |context|.
   // |user_id| identifies the user for which the certificate was requested.
   // |operation_success| is true iff the certificate request operation
   // succeeded.  |certificate| holds the certificate for the platform key on
   // success.  If the certificate request was successful, this method invokes a
   // request to sign the challenge.  If the operation timed out prior to this
   // method being called, this method does nothing - notably, the callback is
   // not invoked.
   void OnCertificateReady(const ChallengeContext& context,
-                          const std::string& user_id,
+                          const user_manager::UserID& user_id,
                           scoped_ptr<base::Timer> timer,
                           bool operation_success,
                           const std::string& certificate);
 
   // A callback run after a constant delay to handle timeouts for lengthy
   // certificate requests.  |context.callback| will be invoked with a TIMEOUT
   // result.
   void OnCertificateTimeout(const ChallengeContext& context);
 
   // A callback called when a challenge signing request has completed.  The
@@ skipping 21 matching lines @@
   scoped_ptr<Delegate> default_delegate_;
   base::TimeDelta timeout_delay_;
 
   DISALLOW_COPY_AND_ASSIGN(PlatformVerificationFlow);
 };
 
 }  // namespace attestation
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_ATTESTATION_PLATFORM_VERIFICATION_FLOW_H_