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

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

Index: chrome/browser/chromeos/login/signin/oauth2_login_manager.h
diff --git a/chrome/browser/chromeos/login/signin/oauth2_login_manager.h b/chrome/browser/chromeos/login/signin/oauth2_login_manager.h
index bac1bcb0b339eb316a4b501be2f39c2653cae824..1d5b04c4605f56fdb77e45d4bc3b2668a6fce541 100644
--- a/chrome/browser/chromeos/login/signin/oauth2_login_manager.h
+++ b/chrome/browser/chromeos/login/signin/oauth2_login_manager.h
@@ -157,7 +157,7 @@ class OAuth2LoginManager : public KeyedService,
   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.