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

chrome/browser/ui/webui/chromeos/login/authenticated_user_email_retriever.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/ui/webui/chromeos/login/authenticated_user_email_retriever.h"
 
 #include <utility>
 #include <vector>
 
 #include "google_apis/gaia/gaia_auth_util.h"
@@ skipping 11 matching lines @@
                          GaiaConstants::kChromeSource,
                          url_request_context_getter.get()) {
   gaia_auth_fetcher_.StartListAccounts();
 }
 
 AuthenticatedUserEmailRetriever::~AuthenticatedUserEmailRetriever() {
 }
 
 void AuthenticatedUserEmailRetriever::OnListAccountsSuccess(
     const std::string& data) {
-  std::vector<std::pair<std::string, bool> > accounts;
+  std::vector<gaia::ListedAccount> accounts;
   gaia::ParseListAccountsData(data, &accounts);
   if (accounts.size() != 1)
     callback_.Run(std::string());
   else
-    callback_.Run(accounts.front().first);
+    callback_.Run(accounts.front().email);
 }
 
 void AuthenticatedUserEmailRetriever::OnListAccountsFailure(
     const GoogleServiceAuthError& error) {
   callback_.Run(std::string());
 }
 
 }  // namespace chromeos