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

chrome/browser/chromeos/attestation/platform_verification_flow.cc

Index: chrome/browser/chromeos/attestation/platform_verification_flow.cc
diff --git a/chrome/browser/chromeos/attestation/platform_verification_flow.cc b/chrome/browser/chromeos/attestation/platform_verification_flow.cc
index 03170f4fec517af869b79878987f49e8d5871eeb..9c7ad7b8a58cc829df7d81c1b2c2da887d5cf045 100644
--- a/chrome/browser/chromeos/attestation/platform_verification_flow.cc
+++ b/chrome/browser/chromeos/attestation/platform_verification_flow.cc
@@ -26,6 +26,7 @@
 #include "components/content_settings/core/common/content_settings_pattern.h"
 #include "components/content_settings/core/common/permission_request_id.h"
 #include "components/user_manager/user.h"
+#include "components/user_manager/user_id.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/render_process_host.h"
@@ -238,11 +239,11 @@ void PlatformVerificationFlow::OnAttestationPrepared(
     return;
   }
 
-  GetCertificate(context, user->email(), false /* Don't force a new key */);
+  GetCertificate(context, user->GetUserID(), false /* Don't force a new key */);
 }
 
 void PlatformVerificationFlow::GetCertificate(const ChallengeContext& context,
-                                              const std::string& user_id,
+                                              const user_manager::UserID& user_id,
                                               bool force_new_key) {
   scoped_ptr<base::Timer> timer(new base::Timer(false,    // Don't retain.
                                                 false));  // Don't repeat.
@@ -268,7 +269,7 @@ void PlatformVerificationFlow::GetCertificate(const ChallengeContext& context,
 
 void PlatformVerificationFlow::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) {