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

components/signin/core/browser/about_signin_internals.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 COMPONENTS_SIGNIN_CORE_BROWSER_ABOUT_SIGNIN_INTERNALS_H_
 #define COMPONENTS_SIGNIN_CORE_BROWSER_ABOUT_SIGNIN_INTERNALS_H_
 
 #include <map>
 #include <string>
 
@@ skipping 72 matching lines @@
   //        "data": [List of {"label" : "foo-field", "value" : "foo"} elems]
   //       }],
   //   "token_info" :
   //     [ List of {"name": "foo-name", "token" : "foo-token",
   //                 "status": "foo_stat", "time" : "foo_time"} elems]
   //  }
   scoped_ptr<base::DictionaryValue> GetSigninStatus();
 
   // GaiaCookieManagerService::Observer implementations.
   void OnGaiaAccountsInCookieUpdated(
-      const std::vector<std::pair<std::string, bool> >& gaia_accounts,
+      const std::vector<gaia::ListedAccount>& gaia_accounts,
       const GoogleServiceAuthError& error) override;
 
  private:
   // Encapsulates diagnostic information about tokens for different services.
   struct TokenInfo {
     TokenInfo(const std::string& consumer_id,
               const OAuth2TokenService::ScopeSet& scopes);
     ~TokenInfo();
     base::DictionaryValue* ToValue() const;
 
@@ skipping 105 matching lines @@
   // Encapsulates the actual signin and token related values.
   // Most of the values are mirrored in the prefs for persistence.
   SigninStatus signin_status_;
 
   base::ObserverList<Observer> signin_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(AboutSigninInternals);
 };
 
 #endif  // COMPONENTS_SIGNIN_CORE_BROWSER_ABOUT_SIGNIN_INTERNALS_H_