cros: Break BaseScreenHandler into two classes.

chrome/browser/ui/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//chrome/common/features.gni")
@@ skipping 221 matching lines @@
     "webui/chromeos/login/app_launch_splash_screen_handler.cc",
     "webui/chromeos/login/app_launch_splash_screen_handler.h",
     "webui/chromeos/login/arc_kiosk_splash_screen_handler.cc",
     "webui/chromeos/login/arc_kiosk_splash_screen_handler.h",
     "webui/chromeos/login/arc_terms_of_service_screen_handler.cc",
     "webui/chromeos/login/arc_terms_of_service_screen_handler.h",
     "webui/chromeos/login/auto_enrollment_check_screen_handler.cc",
     "webui/chromeos/login/auto_enrollment_check_screen_handler.h",
     "webui/chromeos/login/base_screen_handler.cc",
     "webui/chromeos/login/base_screen_handler.h",
+    "webui/chromeos/login/base_webui_handler.cc",
+    "webui/chromeos/login/base_webui_handler.h",
     "webui/chromeos/login/controller_pairing_screen_handler.cc",
     "webui/chromeos/login/controller_pairing_screen_handler.h",
     "webui/chromeos/login/core_oobe_handler.cc",
     "webui/chromeos/login/core_oobe_handler.h",
     "webui/chromeos/login/device_disabled_screen_handler.cc",
     "webui/chromeos/login/device_disabled_screen_handler.h",
     "webui/chromeos/login/enable_debugging_screen_handler.cc",
     "webui/chromeos/login/enable_debugging_screen_handler.h",
     "webui/chromeos/login/enrollment_screen_handler.cc",
     "webui/chromeos/login/enrollment_screen_handler.h",
@@ skipping 3277 matching lines @@
       "test/test_confirm_bubble_model.cc",
       "test/test_confirm_bubble_model.h",
     ]
     deps += [ "//chrome/test:test_support_ui" ]
   }
 
   if (enable_extensions) {
     deps += [ "//extensions/browser" ]
   }
 }