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..40c0cc534286c7729fca064708a2290e815518ce 100644
--- a/chrome/browser/chromeos/login/screens/update_screen.cc
+++ b/chrome/browser/chromeos/login/screens/update_screen.cc
@@ -11,6 +11,7 @@
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
 #include "base/threading/thread_restrictions.h"
+#include "chrome/browser/chromeos/login/screen_manager.h"
 #include "chrome/browser/chromeos/login/screens/error_screen.h"
 #include "chrome/browser/chromeos/login/screens/screen_observer.h"
 #include "chrome/browser/chromeos/login/screens/update_screen_actor.h"
@@ -84,6 +85,12 @@ 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)