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

chrome/browser/chromeos/attestation/platform_verification_flow.h

Index: chrome/browser/chromeos/attestation/platform_verification_flow.h
diff --git a/chrome/browser/chromeos/attestation/platform_verification_flow.h b/chrome/browser/chromeos/attestation/platform_verification_flow.h
index eaa6abb1c7a2e87120242e1b17c193c3c597355e..4d33ca53a903b262da996cdf962d194cb651c593 100644
--- a/chrome/browser/chromeos/attestation/platform_verification_flow.h
+++ b/chrome/browser/chromeos/attestation/platform_verification_flow.h
@@ -25,6 +25,7 @@ class AsyncMethodCaller;
 
 namespace user_manager {
 class User;
+class UserID;
 }
 
 namespace chromeos {
@@ -169,7 +170,7 @@ class PlatformVerificationFlow
   // 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
@@ -182,7 +183,7 @@ class PlatformVerificationFlow
   // 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);