aura-mus: Implement DesktopWindowTreeHostMus::StackAbove().

ui/aura/mus/window_tree_host_mus_delegate.h

Index: ui/aura/mus/window_tree_host_mus_delegate.h
diff --git a/ui/aura/mus/window_tree_host_mus_delegate.h b/ui/aura/mus/window_tree_host_mus_delegate.h
index b25416dd7f75daabdd5ee02fa1f09fa45e495354..1d4d0113c6862d0ad0352c1807a46dc099887bc8 100644
--- a/ui/aura/mus/window_tree_host_mus_delegate.h
+++ b/ui/aura/mus/window_tree_host_mus_delegate.h
@@ -46,6 +46,11 @@ class AURA_EXPORT WindowTreeHostMusDelegate {
   virtual void OnWindowTreeHostDeactivateWindow(
       WindowTreeHostMus* window_tree_host) = 0;
 
+  // Called to stack the native window above the native window of |window|.
+  virtual void OnWindowTreeHostStackAbove(
+      WindowTreeHostMus* window_tree_host,
+      Window* window) = 0;
+
   // Called to stack the native window above other native windows.
   virtual void OnWindowTreeHostStackAtTop(
       WindowTreeHostMus* window_tree_host) = 0;