WIP: Propagate SurfaceID up window tree hierarchy

ash/mus/bridge/wm_window_mus.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/mus/bridge/wm_window_mus.h"
 
 #include "ash/common/wm/container_finder.h"
 #include "ash/common/wm/window_positioning_utils.h"
 #include "ash/common/wm/window_state.h"
 #include "ash/common/wm_layout_manager.h"
@@ skipping 426 matching lines @@
   return WmShellMus::GetToplevelAncestor(window_);
 }
 
 void WmWindowMus::SetParentUsingContext(WmWindow* context,
                                         const gfx::Rect& screen_bounds) {
   wm::GetDefaultParent(context, this, screen_bounds)->AddChild(this);
 }
 
 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) {
   window_->RemoveChild(GetMusWindow(child));
 }
 
 const WmWindow* WmWindowMus::GetParent() const {
   return Get(window_->parent());
 }
 
@@ skipping 472 matching lines @@
 }
 
 void WmWindowMus::OnTransientChildRemoved(ui::Window* window,
                                           ui::Window* transient) {
   FOR_EACH_OBSERVER(WmTransientWindowObserver, transient_observers_,
                     OnTransientChildRemoved(this, Get(transient)));
 }
 
 }  // namespace mus
 }  // namespace ash