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

components/signin/core/browser/gaia_cookie_manager_service.cc

Index: components/signin/core/browser/gaia_cookie_manager_service.cc
diff --git a/components/signin/core/browser/gaia_cookie_manager_service.cc b/components/signin/core/browser/gaia_cookie_manager_service.cc
index 0926687da4838a9f494fbb3bfde11bd438dbbd99..6da9051724a0b17b0d14c041e8451357534987e5 100644
--- a/components/signin/core/browser/gaia_cookie_manager_service.cc
+++ b/components/signin/core/browser/gaia_cookie_manager_service.cc
@@ -14,9 +14,9 @@
 #include "base/strings/stringprintf.h"
 #include "base/time/time.h"
 #include "base/values.h"
+#include "components/signin/core/browser/account_tracker_service.h"
 #include "components/signin/core/browser/signin_metrics.h"
 #include "google_apis/gaia/gaia_auth_fetcher.h"
-#include "google_apis/gaia/gaia_auth_util.h"
 #include "google_apis/gaia/gaia_constants.h"
 #include "google_apis/gaia/gaia_urls.h"
 #include "google_apis/gaia/oauth2_token_service.h"
@@ -338,7 +338,7 @@ void GaiaCookieManagerService::AddAccountToCookieWithToken(
 }
 
 bool GaiaCookieManagerService::ListAccounts(
-    std::vector<std::pair<std::string,bool> >* accounts) {
+    std::vector<gaia::ListedAccount>* accounts) {
   DCHECK(accounts);
   accounts->clear();
 
@@ -560,6 +560,11 @@ void GaiaCookieManagerService::OnListAccountsSuccess(const std::string& data) {
     return;
   }
 
+  for (gaia::ListedAccount account : listed_accounts_) {
+    account.id = AccountTrackerService::PickAccountIdForAccount(
+        signin_client_->GetPrefs(), account.gaia_id, account.email);
+  }
+
   list_accounts_fetched_once_ = true;
   HandleNextRequest();
   // HandleNextRequest before sending out the notification because some