Move all typecasting code to individual screens.

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

Index: chrome/browser/chromeos/login/screens/network_screen.h
diff --git a/chrome/browser/chromeos/login/screens/network_screen.h b/chrome/browser/chromeos/login/screens/network_screen.h
index da50a476978986af51c397fd02145a2179820ddc..1124bb123666586ea367c9580043390bd64940d1 100644
--- a/chrome/browser/chromeos/login/screens/network_screen.h
+++ b/chrome/browser/chromeos/login/screens/network_screen.h
@@ -15,6 +15,8 @@
 
 namespace chromeos {
 
+class ScreenManager;
+
 namespace login {
 class NetworkStateHelper;
 }  // namespace login
@@ -26,6 +28,8 @@ class NetworkScreen : public WizardScreen,
   NetworkScreen(ScreenObserver* screen_observer, NetworkScreenActor* actor);
   virtual ~NetworkScreen();
 
+  static NetworkScreen* Get(ScreenManager* manager);
+
   // WizardScreen implementation:
   virtual void PrepareToShow() OVERRIDE;
   virtual void Show() OVERRIDE;