cros: Use first unlock user's pref for locking screen.

chrome/browser/chromeos/login/user_manager_impl.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chromeos/login/user_manager_impl.h"
 
 #include <cstddef>
 #include <set>
 
 #include "base/bind.h"
@@ skipping 284 matching lines @@
     temp_single_logged_in_users_.clear();
     temp_single_logged_in_users_.insert(temp_single_logged_in_users_.begin(),
                                         active_user_);
     return temp_single_logged_in_users_;
   }
   return lru_logged_in_users_;
 }
 
 UserList UserManagerImpl::GetUnlockUsers() const {
   UserList unlock_users;
-  CHECK(primary_user_);
-  unlock_users.push_back(primary_user_);
+  if (primary_user_)
+    unlock_users.push_back(primary_user_);

[Mr4D (OOO till 08-26), 2013/11/23 00:52:26]

Just checking: So only one user can unlock the session? Somehow I always thought
that - if no enterprise settings were given - every user could do that?

   return unlock_users;
 }
 
 const std::string& UserManagerImpl::GetOwnerEmail() {
   return owner_email_;
 }
 
 void UserManagerImpl::UserLoggedIn(const std::string& user_id,
                                    const std::string& username_hash,
                                    bool browser_restart) {
@@ skipping 1518 matching lines @@
   }
 }
 
 void UserManagerImpl::OnUserNotAllowed() {
   LOG(ERROR) << "Shutdown session because a user is not allowed to be in the "
                 "current session";
   chrome::AttemptUserExit();
 }
 
 }  // namespace chromeos