aura-mus: Implement stacking in DesktopWindowTreeHostMus.

ui/aura/mus/window_tree_client_delegate.h

Index: ui/aura/mus/window_tree_client_delegate.h
diff --git a/ui/aura/mus/window_tree_client_delegate.h b/ui/aura/mus/window_tree_client_delegate.h
index 609a05785854c6fd8accfccc5d02dfdd67f333df..11446a736b6e05864033e07b9f5f9d2a89d3697d 100644
--- a/ui/aura/mus/window_tree_client_delegate.h
+++ b/ui/aura/mus/window_tree_client_delegate.h
@@ -18,6 +18,7 @@ class PropertyConverter;
 class Window;
 class WindowTreeClient;
 class WindowTreeHostMus;
+enum class ChangeType;
 
 namespace client {
 class CaptureClient;
@@ -62,6 +63,9 @@ class AURA_EXPORT WindowTreeClientDelegate {
   virtual void OnPointerEventObserved(const ui::PointerEvent& event,
                                       Window* target) = 0;
 
+  // Called during change completion. Implemented in testing delegates.
+  virtual void OnChangeCompleted(ChangeType type, bool success) {}
+
   // Mus expects a single CaptureClient is used for all WindowTreeHosts. This
   // returns it. GetCaptureClient() is called from the constructor.
   virtual client::CaptureClient* GetCaptureClient() = 0;