Made a ScopedMockUserManagerEnabler to simplify UserManager mocking life-cycle.

chrome/browser/chromeos/login/mock_user_manager.cc

 // Copyright (c) 2012 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/mock_user_manager.h"
 
 namespace chromeos {
 
 MockUserManager::MockUserManager() : user_(NULL) {}
 
 MockUserManager::~MockUserManager() {
   delete user_;
 }
 
 const User& MockUserManager::GetLoggedInUser() const {
   return *user_;
 }
 
 User& MockUserManager::GetLoggedInUser() {
   return *user_;
 }
 
 // Creates a new User instance.
 void MockUserManager::SetLoggedInUser(const std::string& email, bool guest) {
   delete user_;
   user_ = new User(email, guest);
 }
 
+ScopedMockUserManagerEnabler::ScopedMockUserManagerEnabler() {
+  user_manager_.reset(new MockUserManager());
+  old_user_manager_ = UserManager::Set(user_manager_.get());
+}
+
+ScopedMockUserManagerEnabler::~ScopedMockUserManagerEnabler() {
+  UserManager::Set(old_user_manager_);
+}
+
+MockUserManager* ScopedMockUserManagerEnabler::user_manager() {
+  return user_manager_.get();
+}
+
 }  // namespace chromeos