mus: Changes SetDisplayRoot() to create actual display

services/ui/display/screen_manager_forwarding.h

Index: services/ui/display/screen_manager_forwarding.h
diff --git a/services/ui/display/screen_manager_forwarding.h b/services/ui/display/screen_manager_forwarding.h
index d3a0f5bb4671b5ecb45f5502c95f19ddb628863a..9eac7837521c878a21b4be9033f638db0ffda66a 100644
--- a/services/ui/display/screen_manager_forwarding.h
+++ b/services/ui/display/screen_manager_forwarding.h
@@ -36,6 +36,7 @@ class ScreenManagerForwarding
   void AddInterfaces(service_manager::BinderRegistry* registry) override;
   void Init(ScreenManagerDelegate* delegate) override;
   void RequestCloseDisplay(int64_t display_id) override;
+  display::ScreenBase* GetScreen() override;
 
   // NativeDisplayObserver:
   void OnConfigurationChanged() override;
@@ -80,6 +81,7 @@ class ScreenManagerForwarding
       const mojom::NativeDisplayDelegate::ConfigureCallback& callback,
       bool status);
 
+  std::unique_ptr<display::ScreenBase> screen_;
   mojo::Binding<mojom::NativeDisplayDelegate> binding_;
   mojom::NativeDisplayObserverPtr observer_;