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

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

Index: chrome/browser/chromeos/login/signin/oauth2_login_manager.cc
diff --git a/chrome/browser/chromeos/login/signin/oauth2_login_manager.cc b/chrome/browser/chromeos/login/signin/oauth2_login_manager.cc
index 890bd89ac1bf289dce49121aae4e8b281c0e8f79..6b3795fc588fcd4d49a27057c90e2e45a90de97e 100644
--- a/chrome/browser/chromeos/login/signin/oauth2_login_manager.cc
+++ b/chrome/browser/chromeos/login/signin/oauth2_login_manager.cc
@@ -307,18 +307,18 @@ void OAuth2LoginManager::OnSessionMergeFailure(bool connection_error) {
 }
 
 void OAuth2LoginManager::OnListAccountsSuccess(
-    const std::vector<std::pair<std::string, bool>>& accounts) {
+    const std::vector<gaia::ListedAccount>& accounts) {
   MergeVerificationOutcome outcome = POST_MERGE_SUCCESS;
   // Let's analyze which accounts we see logged in here:
   std::string user_email = gaia::CanonicalizeEmail(GetPrimaryAccountId());
   if (!accounts.empty()) {
     bool found = false;
     bool first = true;
-    for (std::vector<std::pair<std::string, bool> >::const_iterator iter =
+    for (std::vector<gaia::ListedAccount>::const_iterator iter =
              accounts.begin();
          iter != accounts.end(); ++iter) {
-      if (gaia::CanonicalizeEmail(iter->first) == user_email) {
-        found = iter->second;
+      if (gaia::CanonicalizeEmail(iter->email) == user_email) {

[xiyuan, 2015/06/04 15:59:24]

nit: |email| is canonicalized so we probably can remove CanonicalizeEmail call
here.

[Mike Lerman, 2015/06/04 16:52:41]

Done.

+        found = iter->valid;
         break;
       }