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

ui/views/mus/desktop_window_tree_host_mus.cc

Index: ui/views/mus/desktop_window_tree_host_mus.cc
diff --git a/ui/views/mus/desktop_window_tree_host_mus.cc b/ui/views/mus/desktop_window_tree_host_mus.cc
index 9cff6ce99ca63f8bbdb57d97e12ceb2b82202b7f..02cf6376934d0bff1123c9b9149b382b160d6280 100644
--- a/ui/views/mus/desktop_window_tree_host_mus.cc
+++ b/ui/views/mus/desktop_window_tree_host_mus.cc
@@ -406,9 +406,12 @@ void DesktopWindowTreeHostMus::SetSize(const gfx::Size& size) {
   SetBoundsInDIP(screen_bounds);
 }
 
-void DesktopWindowTreeHostMus::StackAbove(aura::Window* window) {
-  // TODO: implement window stacking, http://crbug.com/663617.
-  NOTIMPLEMENTED();
+void DesktopWindowTreeHostMus::StackAbove(aura::Window* relative) {
+  // Windows and X11 check for |relative| being nullptr and fail silently. It
+  // also looks like |relative| is usually multiple children deep in the root
+  // window, which we must pass instead.
+  if (relative && relative->GetRootWindow())
+    WindowTreeHostMus::StackAbove(relative->GetRootWindow());
 }
 
 void DesktopWindowTreeHostMus::StackAtTop() {