cros: Break BaseScreenHandler into two classes.

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

 // Copyright 2017 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_GAIA_VIEW_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_GAIA_VIEW_H_
 
 #include <string>
 
 #include "base/bind.h"
 #include "base/compiler_specific.h"
 #include "base/macros.h"
+#include "chrome/browser/chromeos/login/oobe_screen.h"
 
 namespace chromeos {
 
 class GaiaView {
  public:
+  constexpr static OobeScreen kScreenId = OobeScreen::SCREEN_GAIA_SIGNIN;
+
   GaiaView() = default;
   virtual ~GaiaView() = default;
 
   // Decides whether an auth extension should be pre-loaded. If it should,
   // pre-loads it.
   virtual void MaybePreloadAuthExtension() = 0;
 
   virtual void DisableRestrictiveProxyCheckForTest() = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(GaiaView);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_GAIA_VIEW_H_