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

chrome/browser/signin/fake_profile_oauth2_token_service.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_SIGNIN_FAKE_PROFILE_OAUTH2_TOKEN_SERVICE_H_
 #define CHROME_BROWSER_SIGNIN_FAKE_PROFILE_OAUTH2_TOKEN_SERVICE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 134 matching lines @@
                         const ScopeSet& scopes,
                         const GoogleServiceAuthError& error,
                         const std::string& access_token,
                         const base::Time& expiration);
 
   std::string GetRefreshToken(const std::string& account_id) const;
 
   std::vector<PendingRequest> pending_requests_;
 
   // Maps account ids to their refresh token strings.
-  std::map<std::string, std::string> refresh_tokens_;
+  std::map<user_manager::UserID, std::string> refresh_tokens_;
 
   // If true, then this fake service will post responses to
   // |FetchOAuth2Token| on the current run loop. There is no need to call
   // |IssueTokenForScope| in this case.
   bool auto_post_fetch_response_on_message_loop_;
 
   base::WeakPtrFactory<FakeProfileOAuth2TokenService> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(FakeProfileOAuth2TokenService);
 };
 
 #endif  // CHROME_BROWSER_SIGNIN_FAKE_PROFILE_OAUTH2_TOKEN_SERVICE_H_