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

chrome/browser/supervised_user/legacy/supervised_user_refresh_token_fetcher.cc

Index: chrome/browser/supervised_user/legacy/supervised_user_refresh_token_fetcher.cc
diff --git a/chrome/browser/supervised_user/legacy/supervised_user_refresh_token_fetcher.cc b/chrome/browser/supervised_user/legacy/supervised_user_refresh_token_fetcher.cc
index ed4e0f4b0b8d8d9516013665cbeb178121e87ca4..9ce59c49a357151ef3477b9dd3b3ca67b9c4e7e5 100644
--- a/chrome/browser/supervised_user/legacy/supervised_user_refresh_token_fetcher.cc
+++ b/chrome/browser/supervised_user/legacy/supervised_user_refresh_token_fetcher.cc
@@ -9,6 +9,7 @@
 #include "base/logging.h"
 #include "base/strings/stringprintf.h"
 #include "base/values.h"
+#include "components/user_manager/user_id.h"
 #include "google_apis/gaia/gaia_constants.h"
 #include "google_apis/gaia/gaia_oauth_client.h"
 #include "google_apis/gaia/gaia_urls.h"
@@ -63,7 +64,7 @@ class SupervisedUserRefreshTokenFetcherImpl
   ~SupervisedUserRefreshTokenFetcherImpl() override;
 
   // SupervisedUserRefreshTokenFetcher implementation:
-  void Start(const std::string& supervised_user_id,
+  void Start(const user_manager::UserID& supervised_user_id,
              const std::string& device_name,
              const TokenCallback& callback) override;
 
@@ -101,7 +102,7 @@ class SupervisedUserRefreshTokenFetcherImpl
   URLRequestContextGetter* context_;
 
   std::string device_name_;
-  std::string supervised_user_id_;
+  user_manager::UserID supervised_user_id_;
   TokenCallback callback_;
 
   scoped_ptr<OAuth2TokenService::Request> access_token_request_;
@@ -121,13 +122,14 @@ SupervisedUserRefreshTokenFetcherImpl::SupervisedUserRefreshTokenFetcherImpl(
       account_id_(account_id),
       device_id_(device_id),
       context_(context),
+      supervised_user_id_(std::string(), std::string()),
       access_token_expired_(false) {}
 
 SupervisedUserRefreshTokenFetcherImpl::
 ~SupervisedUserRefreshTokenFetcherImpl() {}
 
 void SupervisedUserRefreshTokenFetcherImpl::Start(
-    const std::string& supervised_user_id,
+    const user_manager::UserID& supervised_user_id,
     const std::string& device_name,
     const TokenCallback& callback) {
   DCHECK(callback_.is_null());
@@ -170,7 +172,7 @@ void SupervisedUserRefreshTokenFetcherImpl::OnGetTokenSuccess(
       net::EscapeUrlEncodedData(
           GaiaUrls::GetInstance()->oauth2_chrome_client_id(), true).c_str(),
       net::EscapeUrlEncodedData(kChromeSyncSupervisedOAuth2Scope, true).c_str(),
-      net::EscapeUrlEncodedData(supervised_user_id_, true).c_str(),
+      net::EscapeUrlEncodedData(supervised_user_id_.GetUserEmail(), true).c_str(),
       net::EscapeUrlEncodedData(device_name_, true).c_str());
   if (!device_id_.empty()) {
     body.append(base::StringPrintf(