Implement users options handling in login screen.

chrome/browser/chromeos/login/user_manager.cc

Index: chrome/browser/chromeos/login/user_manager.cc
diff --git a/chrome/browser/chromeos/login/user_manager.cc b/chrome/browser/chromeos/login/user_manager.cc
index 3336622e8ce049afcd7c8196fe35e1cf1c76c743..571871df7a3df081def734ca2bc6f28af6b3ef23 100644
--- a/chrome/browser/chromeos/login/user_manager.cc
+++ b/chrome/browser/chromeos/login/user_manager.cc
@@ -214,6 +214,18 @@ void UserManager::RemoveUser(const std::string& email) {
   prefs->SavePersistentPrefs();
 }
 
+bool UserManager::IsKnownUser(const std::string& email) {
+  std::vector<User> users = GetUsers();
+  for (std::vector<User>::iterator it = users.begin();
+       it < users.end();
+       ++it) {
+    if (it->email() == email)
+      return true;
+  }
+
+  return false;
+}
+
 void UserManager::SetLoggedInUserImage(const SkBitmap& image) {
   if (logged_in_user_.email().empty())
     return;