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

chrome/browser/chromeos/policy/policy_cert_service_factory.h

Index: chrome/browser/chromeos/policy/policy_cert_service_factory.h
diff --git a/chrome/browser/chromeos/policy/policy_cert_service_factory.h b/chrome/browser/chromeos/policy/policy_cert_service_factory.h
index b2851ba25b04071277765d05c09c61161aa32a06..e21b4bb8cf1acbc2ea27f831d1ec0954a8b3add1 100644
--- a/chrome/browser/chromeos/policy/policy_cert_service_factory.h
+++ b/chrome/browser/chromeos/policy/policy_cert_service_factory.h
@@ -5,8 +5,6 @@
 #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"
@@ -17,6 +15,10 @@ template <typename T> struct DefaultSingletonTraits;
 class PrefRegistrySimple;
 class Profile;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace policy {
 
 class PolicyCertService;
@@ -42,9 +44,9 @@ class PolicyCertServiceFactory : public BrowserContextKeyedServiceFactory {
 
   // 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);