mus: Changes SetDisplayRoot() to create actual display

services/ui/display/screen_manager.h

Index: services/ui/display/screen_manager.h
diff --git a/services/ui/display/screen_manager.h b/services/ui/display/screen_manager.h
index 9252194c689f0c9ee2bff131f947593f9da64381..017b40db9addb8983baadbc00ce2932fa17bc131 100644
--- a/services/ui/display/screen_manager.h
+++ b/services/ui/display/screen_manager.h
@@ -16,6 +16,8 @@ class BinderRegistry;
 
 namespace display {
 
+class ScreenBase;
+
 // ScreenManager provides the necessary functionality to configure all
 // attached physical displays.
 class ScreenManager {
@@ -40,6 +42,8 @@ class ScreenManager {
   // Handle requests from the platform to close a display.
   virtual void RequestCloseDisplay(int64_t display_id) = 0;
 
+  virtual display::ScreenBase* GetScreen() = 0;
+
  private:
   static ScreenManager* instance_;  // Instance is not owned.