Move all typecasting code to individual screens.

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

Index: chrome/browser/chromeos/login/screens/user_image_screen.cc
diff --git a/chrome/browser/chromeos/login/screens/user_image_screen.cc b/chrome/browser/chromeos/login/screens/user_image_screen.cc
index bd709db795b5b4a1080aba28b7ecd35f8ede4ff3..bb34c8a8fac8a7dccd7f979c38489b03f5454837 100644
--- a/chrome/browser/chromeos/login/screens/user_image_screen.cc
+++ b/chrome/browser/chromeos/login/screens/user_image_screen.cc
@@ -57,6 +57,12 @@ const int kSyncTimeoutSeconds = 10;
 
 }  // namespace
 
+// static
+UserImageScreen* UserImageScreen::Get(ScreenManager* manager) {
+  return static_cast<UserImageScreen*>(
+      manager->GetScreen(WizardController::kUserImageScreenName));
+}
+
 UserImageScreen::UserImageScreen(ScreenObserver* screen_observer,
                                  UserImageScreenActor* actor)
     : WizardScreen(screen_observer),