Update add user button dynamically after policy push

Using UpdateUIState() is insufficient because it does not have the
mechanism to change 'add button disabled' state back to 'not disabled'. We
should explicitly call UpdateAddButtonStatus().

BUG=411618

Review-Url: https://codereview.chromium.org/2850723002
Cr-Commit-Position: refs/heads/master@{#467910}

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

@@ -1069,6 +1069,7 @@ void SigninScreenHandler::OnPreferencesChanged() {
     // has changed so that reloaded GAIA shows/hides the option to create a new
     // account.
     UpdateUIState(UI_STATE_ACCOUNT_PICKER, nullptr);
+    UpdateAddButtonStatus();
   }
 }