Implement users options handling in login screen.

chrome/browser/chromeos/login/user_manager.h

Index: chrome/browser/chromeos/login/user_manager.h
diff --git a/chrome/browser/chromeos/login/user_manager.h b/chrome/browser/chromeos/login/user_manager.h
index 67630699d432ce238e650cdcf7234ad7502001a0..d3e40be6111b0cbbbfa3f27399bde174a91b4148 100644
--- a/chrome/browser/chromeos/login/user_manager.h
+++ b/chrome/browser/chromeos/login/user_manager.h
@@ -69,6 +69,9 @@ class UserManager : public UserImageLoader::Delegate,
   // Remove user from persistent list. NOTE: user's data won't be removed.
   void RemoveUser(const std::string& email);
 
+  // Returns true if given user has logged into the device before.
+  bool IsKnownUser(const std::string& email);
+
   // Returns the logged-in user.
   const User& logged_in_user() {
     return logged_in_user_;