mus: Changes SetDisplayRoot() to create actual display

services/ui/ws/window_server_delegate.h

Index: services/ui/ws/window_server_delegate.h
diff --git a/services/ui/ws/window_server_delegate.h b/services/ui/ws/window_server_delegate.h
index 337cc3880e2757f5df8055a089ad27a6239383ae..5256e11bec3351e10b4e5947965b67d6c4975792 100644
--- a/services/ui/ws/window_server_delegate.h
+++ b/services/ui/ws/window_server_delegate.h
@@ -53,6 +53,12 @@ class WindowServerDelegate {
       mojom::WindowTreeRequest* tree_request,
       mojom::WindowTreeClientPtr* client);
 
+  // Called prior to a new WindowTree being created for a
+  // WindowManagerWindowTreeFactory. |automatically_create_display_roots|
+  // mirrors that of CreateWindowTree(). See it for details.
+  virtual void OnWillCreateTreeForWindowManager(
+      bool automatically_create_display_roots);
+
  protected:
   virtual ~WindowServerDelegate() {}
 };