cros: Break BaseScreenHandler into two classes.

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

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_UPDATE_VIEW_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_UPDATE_VIEW_H_
 
 #include "base/strings/string16.h"
+#include "chrome/browser/chromeos/login/oobe_screen.h"
 
 namespace chromeos {
 
 class UpdateScreen;
 
 // Interface for dependency injection between NetworkScreen and its actual
 // representation. Owned by UpdateScreen.
 class UpdateView {
  public:
+  constexpr static OobeScreen kScreenId = OobeScreen::SCREEN_OOBE_UPDATE;
+
   virtual ~UpdateView() {}
 
   // Shows the contents of the screen.
   virtual void Show() = 0;
 
   // Hides the contents of the screen.
   virtual void Hide() = 0;
 
   // Binds |screen| to the view.
   virtual void Bind(UpdateScreen* screen) = 0;
 
   // Unbinds the screen from the view.
   virtual void Unbind() = 0;
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_UPDATE_VIEW_H_