ListAccounts will return the Gaia ID as well as the email of the account.

chrome/browser/chromeos/login/signin/oauth2_login_manager.h

 // Copyright 2014 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_CHROMEOS_LOGIN_SIGNIN_OAUTH2_LOGIN_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SIGNIN_OAUTH2_LOGIN_MANAGER_H_
 
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
@@ skipping 139 matching lines @@
                               int expires_in_seconds) override;
   void OnGetUserInfoResponse(
       scoped_ptr<base::DictionaryValue> user_info) override;
   void OnOAuthError() override;
   void OnNetworkError(int response_code) override;
 
   // OAuth2LoginVerifier::Delegate overrides.
   void OnSessionMergeSuccess() override;
   void OnSessionMergeFailure(bool connection_error) override;
   void OnListAccountsSuccess(
-      const std::vector<std::pair<std::string, bool>>& accounts) override;
+      const std::vector<gaia::ListedAccount>& accounts) override;
   void OnListAccountsFailure(bool connection_error) override;
 
   // OAuth2TokenFetcher::Delegate overrides.
   void OnOAuth2TokensAvailable(
       const GaiaAuthConsumer::ClientOAuthResult& oauth2_tokens) override;
   void OnOAuth2TokensFetchFailed() override;
 
   // OAuth2TokenService::Observer implementation:
   void OnRefreshTokenAvailable(const std::string& account_id) override;
 
@@ skipping 78 matching lines @@
   // TODO(zelidrag|gspencer): Figure out how to get rid of ProfileHelper so we
   // can change the line below to base::ObserverList<Observer, true>.
   base::ObserverList<Observer, false> observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(OAuth2LoginManager);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SIGNIN_OAUTH2_LOGIN_MANAGER_H_