cros: Break BaseScreenHandler into two classes.

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

 // Copyright 2016 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_ARC_TERMS_OF_SERVICE_SCREEN_VIEW_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_ARC_TERMS_OF_SERVICE_SCREEN_VIEW_H_
 
 #include <string>
 
 #include "base/macros.h"
+#include "chrome/browser/chromeos/login/oobe_screen.h"
 
 namespace chromeos {
 
 class ArcTermsOfServiceScreenViewObserver;
 
 // Interface for dependency injection between TermsOfServiceScreen and its
 // WebUI representation.
 class ArcTermsOfServiceScreenView {
  public:
+  constexpr static OobeScreen kScreenId =
+      OobeScreen::SCREEN_ARC_TERMS_OF_SERVICE;
+
   virtual ~ArcTermsOfServiceScreenView() = default;
 
   // Adds/Removes observer for view.
   virtual void AddObserver(ArcTermsOfServiceScreenViewObserver* observer) = 0;
   virtual void RemoveObserver(
       ArcTermsOfServiceScreenViewObserver* observer) = 0;
 
   // Shows the contents of the screen.
   virtual void Show() = 0;
 
   // Hides the contents of the screen.
   virtual void Hide() = 0;
 
  protected:
   ArcTermsOfServiceScreenView() = default;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ArcTermsOfServiceScreenView);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_ARC_TERMS_OF_SERVICE_SCREEN_VIEW_H_