Enables passing initial properties to top level window creation

ui/views/mus/window_tree_host_mus.cc

Index: ui/views/mus/window_tree_host_mus.cc
diff --git a/ui/views/mus/window_tree_host_mus.cc b/ui/views/mus/window_tree_host_mus.cc
index 0d08e1b3f41fc1ad4ee81980577e1e255d440d8c..e0ec32fe891991032ae27225d717864c2559a6dc 100644
--- a/ui/views/mus/window_tree_host_mus.cc
+++ b/ui/views/mus/window_tree_host_mus.cc
@@ -23,7 +23,7 @@ bool IsUsingTestContext() {
   return aura::Env::GetInstance()->context_factory()->DoesCreateTestContexts();
 }
 
-}
+}  // namespace
 
 ////////////////////////////////////////////////////////////////////////////////
 // WindowTreeHostMus, public:
@@ -31,6 +31,7 @@ bool IsUsingTestContext() {
 WindowTreeHostMus::WindowTreeHostMus(NativeWidgetMus* native_widget,
                                      ui::Window* window)
     : native_widget_(native_widget) {
+  CreateCompositor();
   gfx::AcceleratedWidget accelerated_widget;
   if (IsUsingTestContext()) {
     accelerated_widget = gfx::kNullAcceleratedWidget;