Made a ScopedMockUserManagerEnabler to simplify UserManager mocking life-cycle.

chrome/browser/chromeos/login/mock_user_manager.cc

Index: chrome/browser/chromeos/login/mock_user_manager.cc
diff --git a/chrome/browser/chromeos/login/mock_user_manager.cc b/chrome/browser/chromeos/login/mock_user_manager.cc
index 4bd67981fd56bc33ec049dc28e293ca4695df1c9..f4e700fa01ca90312041b3586c428fffc21b3f57 100644
--- a/chrome/browser/chromeos/login/mock_user_manager.cc
+++ b/chrome/browser/chromeos/login/mock_user_manager.cc
@@ -26,4 +26,17 @@ void MockUserManager::SetLoggedInUser(const std::string& email, bool guest) {
   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