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

chrome/browser/chromeos/login/signin/oauth2_login_verifier.cc

 // 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.
 
 #include "chrome/browser/chromeos/login/signin/oauth2_login_verifier.h"
 
 #include <vector>
 
 #include "base/logging.h"
 #include "content/public/browser/browser_thread.h"
@@ skipping 15 matching lines @@
   cookie_manager_service_->AddObserver(this);
 }
 
 OAuth2LoginVerifier::~OAuth2LoginVerifier() {
   cookie_manager_service_->RemoveObserver(this);
 }
 
 void OAuth2LoginVerifier::VerifyUserCookies(Profile* profile) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
-  std::vector<std::pair<std::string, bool> > accounts;
+  std::vector<gaia::ListedAccount> accounts;
   if (cookie_manager_service_->ListAccounts(&accounts)) {
     OnGaiaAccountsInCookieUpdated(
         accounts, GoogleServiceAuthError(GoogleServiceAuthError::NONE));
   }
 }
 
 void OAuth2LoginVerifier::VerifyProfileTokens(Profile* profile) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   if (access_token_.empty()) {
     cookie_manager_service_->AddAccountToCookie(primary_account_id_);
@@ skipping 14 matching lines @@
     delegate_->OnSessionMergeSuccess();
     return;
   }
 
   LOG(WARNING) << "Failed MergeSession request,"
                << " error: " << error.state();
   delegate_->OnSessionMergeFailure(error.IsTransientError());
 }
 
 void OAuth2LoginVerifier::OnGaiaAccountsInCookieUpdated(
-      const std::vector<std::pair<std::string, bool> >& accounts,
-      const GoogleServiceAuthError& error) {
+    const std::vector<gaia::ListedAccount>& accounts,
+    const GoogleServiceAuthError& error) {
   if (error.state() == GoogleServiceAuthError::State::NONE) {
     VLOG(1) << "ListAccounts successful.";
     delegate_->OnListAccountsSuccess(accounts);
     return;
   }
 
   LOG(WARNING) << "Failed to get list of session accounts, "
                << " error: " << error.state();
   delegate_->OnListAccountsFailure(error.IsTransientError());
 }
 
 }  // namespace chromeos