mus: Changes SetDisplayRoot() to create actual display

services/ui/display/screen_manager_ozone_external.h

Index: services/ui/display/screen_manager_ozone_external.h
diff --git a/services/ui/display/screen_manager_ozone_external.h b/services/ui/display/screen_manager_ozone_external.h
index 41fe466008feaeda6aa141cca9786a74fdbcfee4..f4af7d4f4f44a83b74f3b3861c17a8f216f62e83 100644
--- a/services/ui/display/screen_manager_ozone_external.h
+++ b/services/ui/display/screen_manager_ozone_external.h
@@ -26,6 +26,9 @@ class ScreenManagerOzoneExternal : public ScreenManager {
   void AddInterfaces(service_manager::BinderRegistry* registry) override;
   void Init(ScreenManagerDelegate* delegate) override;
   void RequestCloseDisplay(int64_t display_id) override;
+  display::ScreenBase* GetScreen() override;
+
+  std::unique_ptr<display::ScreenBase> screen_;
 
   DISALLOW_COPY_AND_ASSIGN(ScreenManagerOzoneExternal);
 };