cros: Break BaseScreenHandler into two classes.

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

Index: chrome/browser/chromeos/login/screens/host_pairing_screen_view.h
diff --git a/chrome/browser/chromeos/login/screens/host_pairing_screen_view.h b/chrome/browser/chromeos/login/screens/host_pairing_screen_view.h
index 64ded6500e0c0b70c5b4cedbb2124b67b3593171..2958bba3b52457f214cde6b8a2c83590a9b5d31e 100644
--- a/chrome/browser/chromeos/login/screens/host_pairing_screen_view.h
+++ b/chrome/browser/chromeos/login/screens/host_pairing_screen_view.h
@@ -6,6 +6,7 @@
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_HOST_PAIRING_SCREEN_VIEW_H_
 
 #include "base/macros.h"
+#include "chrome/browser/chromeos/login/oobe_screen.h"
 
 namespace base {
 class DictionaryValue;
@@ -47,6 +48,8 @@ class HostPairingScreenView {
     virtual void OnViewDestroyed(HostPairingScreenView* view) = 0;
   };
 
+  constexpr static OobeScreen kScreenId = OobeScreen::SCREEN_OOBE_HOST_PAIRING;
+
   HostPairingScreenView();
   virtual ~HostPairingScreenView();