Move all typecasting code to individual screens.

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

Index: chrome/browser/chromeos/login/screens/update_screen.cc
diff --git a/chrome/browser/chromeos/login/screens/update_screen.cc b/chrome/browser/chromeos/login/screens/update_screen.cc
index 036e0b5a0a23d2d099f49b36e6d571ecb1b48ce7..f8fb372b01183d50541fa688a888bde98d883852 100644
--- a/chrome/browser/chromeos/login/screens/update_screen.cc
+++ b/chrome/browser/chromeos/login/screens/update_screen.cc
@@ -84,6 +84,12 @@ bool UpdateScreen::HasInstance(UpdateScreen* inst) {
   return (found != instance_set.end());
 }
 
+// static
+UpdateScreen* UpdateScreen::Get(ScreenManager* manager) {
+  return static_cast<UpdateScreen*>(
+      manager->GetScreen(WizardController::kUpdateScreenName));
+}
+
 UpdateScreen::UpdateScreen(
     ScreenObserver* screen_observer,
     UpdateScreenActor* actor)