Makes RootWindowController window creation in terms of common types

mash/wm/bridge/wm_globals_mus.cc

Index: mash/wm/bridge/wm_globals_mus.cc
diff --git a/mash/wm/bridge/wm_globals_mus.cc b/mash/wm/bridge/wm_globals_mus.cc
index 8f0445e4b6bfc1c676bebeb2ce9f1207354d6f45..01f964933272d5b15b3e56dca032f9e8eeeaceb5 100644
--- a/mash/wm/bridge/wm_globals_mus.cc
+++ b/mash/wm/bridge/wm_globals_mus.cc
@@ -71,6 +71,10 @@ WmRootWindowControllerMus* WmGlobalsMus::GetRootWindowControllerWithDisplayId(
   return nullptr;
 }
 
+ash::wm::WmWindow* WmGlobalsMus::NewContainerWindow() {
+  return WmWindowMus::Get(client_->NewWindow());
+}
+
 ash::wm::WmWindow* WmGlobalsMus::GetFocusedWindow() {
   return WmWindowMus::Get(client_->GetFocusedWindow());
 }