cros: Break BaseScreenHandler into two classes.

chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.cc

Index: chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.cc
diff --git a/chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.cc b/chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.cc
index aa9ce9bee15e3564c3b49ed8727e3eebcbca6791..1596d5a92f7b8a53c4999891aeabceea5bc91f95 100644
--- a/chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.cc
+++ b/chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.cc
@@ -9,7 +9,8 @@
 
 namespace chromeos {
 
-UserBoardScreenHandler::UserBoardScreenHandler() : weak_factory_(this) {}
+UserBoardScreenHandler::UserBoardScreenHandler()
+    : BaseScreenHandler(kScreenId), weak_factory_(this) {}
 
 UserBoardScreenHandler::~UserBoardScreenHandler() {
 }
@@ -95,12 +96,12 @@ void UserBoardScreenHandler::SetAuthType(
 
 void UserBoardScreenHandler::Bind(UserSelectionScreen* screen) {
   screen_ = screen;
-  BaseScreenHandler::SetBaseScreen(screen_);
+  BaseWebUIHandler::SetBaseScreen(screen_);
 }
 
 void UserBoardScreenHandler::Unbind() {
   screen_ = nullptr;
-  BaseScreenHandler::SetBaseScreen(nullptr);
+  BaseWebUIHandler::SetBaseScreen(nullptr);
 }
 
 base::WeakPtr<UserBoardView> UserBoardScreenHandler::GetWeakPtr() {