WIP: Propagate SurfaceID up window tree hierarchy

ash/mus/bridge/wm_window_mus.cc

Index: ash/mus/bridge/wm_window_mus.cc
diff --git a/ash/mus/bridge/wm_window_mus.cc b/ash/mus/bridge/wm_window_mus.cc
index d0234c695586fd97b05db357c1b954d07c1212b0..f0bf6f7f15e9db7a56e40025cdfc1bb5bf98b23f 100644
--- a/ash/mus/bridge/wm_window_mus.cc
+++ b/ash/mus/bridge/wm_window_mus.cc
@@ -444,6 +444,9 @@ void WmWindowMus::SetParentUsingContext(WmWindow* context,
 
 void WmWindowMus::AddChild(WmWindow* window) {
   window_->AddChild(GetMusWindow(window));
+  // If the child window does not have a native widget mus
+  // (and thus does not have an aura window), then we create a layer with its
+  // surface Id.
 }
 
 void WmWindowMus::RemoveChild(WmWindow* child) {