Move all typecasting code to individual screens.

chrome/browser/chromeos/login/screens/update_screen.h

Index: chrome/browser/chromeos/login/screens/update_screen.h
diff --git a/chrome/browser/chromeos/login/screens/update_screen.h b/chrome/browser/chromeos/login/screens/update_screen.h
index 11de336664ace13bd992fc0b324db3ce12539b0f..f828ca03446d8278154eaff7e570cdc420e9e6de 100644
--- a/chrome/browser/chromeos/login/screens/update_screen.h
+++ b/chrome/browser/chromeos/login/screens/update_screen.h
@@ -22,6 +22,7 @@ namespace chromeos {
 
 class ErrorScreen;
 class NetworkState;
+class ScreenManager;
 class ScreenObserver;
 
 // Controller for the update screen. It does not depend on the specific
@@ -35,6 +36,8 @@ class UpdateScreen: public UpdateEngineClient::Observer,
   UpdateScreen(ScreenObserver* screen_observer, UpdateScreenActor* actor);
   virtual ~UpdateScreen();
 
+  static UpdateScreen* Get(ScreenManager* manager);
+
   // Overridden from WizardScreen.
   virtual void PrepareToShow() OVERRIDE;
   virtual void Show() OVERRIDE;