mus: Changes SetDisplayRoot() to create actual display

services/ui/ws/window_manager_window_tree_factory.cc

Index: services/ui/ws/window_manager_window_tree_factory.cc
diff --git a/services/ui/ws/window_manager_window_tree_factory.cc b/services/ui/ws/window_manager_window_tree_factory.cc
index 0f9589b8176fa4dd47c6dd47b43877c19845f420..89711f2de86f19de85353f30fde2af3c09f8d1d5 100644
--- a/services/ui/ws/window_manager_window_tree_factory.cc
+++ b/services/ui/ws/window_manager_window_tree_factory.cc
@@ -7,6 +7,7 @@
 #include "base/bind.h"
 #include "services/ui/ws/window_manager_window_tree_factory_set.h"
 #include "services/ui/ws/window_server.h"
+#include "services/ui/ws/window_server_delegate.h"
 #include "services/ui/ws/window_tree.h"
 
 namespace ui {
@@ -38,6 +39,10 @@ void WindowManagerWindowTreeFactory::CreateWindowTree(
   if (binding_.is_bound())
     binding_.Close();
 
+  window_manager_window_tree_factory_set_->window_server()
+      ->delegate()
+      ->OnWillCreateTreeForWindowManager(automatically_create_display_roots);
+
   SetWindowTree(GetWindowServer()->CreateTreeForWindowManager(
       user_id_, std::move(window_tree_request), std::move(window_tree_client),
       automatically_create_display_roots));