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

google_apis/gaia/gaia_auth_util.h

 // Copyright (c) 2012 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 GOOGLE_APIS_GAIA_GAIA_AUTH_UTIL_H_
 #define GOOGLE_APIS_GAIA_GAIA_AUTH_UTIL_H_
 
 #include <string>
 #include <utility>
 #include <vector>
 
 class GURL;
 
 namespace gaia {
 
+struct ListedAccount {
+  // The account's ID, as per Chrome, will be determined in the
+  // CookieManagerService.
+  std::string id;
+  std::string email;
+  std::string gaia_id;
+  std::string raw_email;
+  bool valid;
+
+  ListedAccount();
+  ~ListedAccount();
+  bool operator==(const ListedAccount& other) const;
+};
+
 // Perform basic canonicalization of |email_address|, taking into account that
 // gmail does not consider '.' or caps inside a username to matter.
 std::string CanonicalizeEmail(const std::string& email_address);
 
 // Returns the canonical form of the given domain.
 std::string CanonicalizeDomain(const std::string& domain);
 
 // Sanitize emails. Currently, it only ensures all emails have a domain by
 // adding gmail.com if no domain is present.
 std::string SanitizeEmail(const std::string& email_address);
 
 // Returns true if the two specified email addresses are the same.  Both
 // addresses are first sanitized and then canonicalized before comparing.
 bool AreEmailsSame(const std::string& email1, const std::string& email2);
 
 // Extract the domain part from the canonical form of the given email.
 std::string ExtractDomainName(const std::string& email);
 
 bool IsGaiaSignonRealm(const GURL& url);
 
 // Parses JSON data returned by /ListAccounts call, returning a vector of
 // email/valid pairs.  An email addresses is considered valid if a passive
 // login would succeed (i.e. the user does not need to reauthenticate).
 // If there an error parsing the JSON, then false is returned.
 bool ParseListAccountsData(
-    const std::string& data,
-    std::vector<std::pair<std::string, bool> >* accounts);
+    const std::string& data, std::vector<ListedAccount>* accounts);
 
 }  // namespace gaia
 
 #endif  // GOOGLE_APIS_GAIA_GAIA_AUTH_UTIL_H_