Move all typecasting code to individual screens.

chrome/browser/chromeos/login/screen_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/screen_manager.h"
+
+namespace chromeos {
+
+ScreenManager::ScreenManager() {
+}
+
+ScreenManager::~ScreenManager() {
+}
+
+WizardScreen* ScreenManager::GetScreen(const std::string& screen_name) {
+  ScreenMap::const_iterator iter = screens_.find(screen_name);
+  if (iter != screens_.end()) {
+    return iter->second.get();
+  }
+  WizardScreen* result = CreateScreen(screen_name);
+  DCHECK(result) << "Can not create screen named " << screen_name;
+  screens_[screen_name] = make_linked_ptr(result);
+  return result;
+}
+
+bool ScreenManager::HasScreen(const std::string& screen_name) {
+  return screens_.count(screen_name) > 0;
+}
+
+}  // namespace chromeos