Invoke SetCurrentScreen(nullptr) before showing the login screen

OnStartSignInScreen() might show update required screen after OOBE is
completed and WizardController::LoginScreenStarted() would then call
hide on the current screen (update required in this case), which is not
what we want. Also, it makes sense, to first reset the current screen
and then show the next screen.

Bug: 1064271
Change-Id: I37de884c65f3621f057602b4fa1678ebee6efc8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2166903
Commit-Queue: Saurabh Nijhara <snijhara@google.com>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762770}

chrome/browser/chromeos/login/ui/login_display_host_common.cc

@@ -116,14 +116,14 @@ void LoginDisplayHostCommon::StartSignInScreen() {
   connector->ScheduleServiceInitialization(
       kPolicyServiceInitializationDelayMilliseconds);
 
-  // Run UI-specific logic.
-  OnStartSignInScreen();
-
   // Inform wizard controller that login screen has started.
   // TODO(crbug.com/1064271): Move this to OnStartSignInScreen().
   if (WizardController::default_controller())
     WizardController::default_controller()->LoginScreenStarted();
 
+  // Run UI-specific logic.
+  OnStartSignInScreen();
+
   // Enable status area after starting sign-in screen, as it may depend on the
   // UI being visible.
   SetStatusAreaVisible(true);