Moves maintaining root_window_for_new_windows_ to Shell

ash/mus/top_level_window_factory.cc

Index: ash/mus/top_level_window_factory.cc
diff --git a/ash/mus/top_level_window_factory.cc b/ash/mus/top_level_window_factory.cc
index 214ae39258b2a360d5dfc93432d8a6bc7cdee87f..d19293c978011329e63cbe7b3958816cc7cbd81f 100644
--- a/ash/mus/top_level_window_factory.cc
+++ b/ash/mus/top_level_window_factory.cc
@@ -6,7 +6,6 @@
 
 #include "ash/common/wm/container_finder.h"
 #include "ash/common/wm/window_state.h"
-#include "ash/common/wm_shell.h"
 #include "ash/common/wm_window.h"
 #include "ash/mus/disconnected_app_handler.h"
 #include "ash/mus/frame/detached_title_area_renderer.h"
@@ -16,6 +15,7 @@
 #include "ash/public/cpp/shell_window_ids.h"
 #include "ash/root_window_controller.h"
 #include "ash/root_window_settings.h"
+#include "ash/shell.h"
 #include "mojo/public/cpp/bindings/type_converter.h"
 #include "services/ui/public/cpp/property_type_converters.h"
 #include "services/ui/public/interfaces/window_manager.mojom.h"
@@ -73,7 +73,7 @@ RootWindowController* GetRootWindowControllerForNewTopLevelWindow(
     }
   }
   return RootWindowController::ForWindow(
-      WmShell::Get()->GetRootWindowForNewWindows()->aura_window());
+      Shell::GetWmRootWindowForNewWindows()->aura_window());
 }
 
 // Returns the bounds for the new window.