mus: Changes SetDisplayRoot() to create actual display

services/ui/public/interfaces/window_manager_constants.mojom

Index: services/ui/public/interfaces/window_manager_constants.mojom
diff --git a/services/ui/public/interfaces/window_manager_constants.mojom b/services/ui/public/interfaces/window_manager_constants.mojom
index 7863f767227a29faf2064b3ec0c325e6856644df..0919edef1f7a475875b92a465140b9dda0f7abf2 100644
--- a/services/ui/public/interfaces/window_manager_constants.mojom
+++ b/services/ui/public/interfaces/window_manager_constants.mojom
@@ -65,3 +65,10 @@ enum WindowType {
   DRAG,
   UNKNOWN,
 };
+
+// Used when the window manager explicitly adds a new display.
+struct WmViewportMetrics {
+  gfx.mojom.Rect bounds_in_pixels;
+  float device_scale_factor;
+  float ui_scale_factor;
+};