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

ui/aura/mus/window_tree_host_mus.cc

Index: ui/aura/mus/window_tree_host_mus.cc
diff --git a/ui/aura/mus/window_tree_host_mus.cc b/ui/aura/mus/window_tree_host_mus.cc
index 2f5497cb1b7a68d6949bd5916cff16571617b8c9..820702a4a4fb6e07a6267d9252bb93be784e0fda 100644
--- a/ui/aura/mus/window_tree_host_mus.cc
+++ b/ui/aura/mus/window_tree_host_mus.cc
@@ -149,6 +149,10 @@ void WindowTreeHostMus::DeactivateWindow() {
   delegate_->OnWindowTreeHostDeactivateWindow(this);
 }
 
+void WindowTreeHostMus::StackAbove(Window* window) {
+  delegate_->OnWindowTreeHostStackAbove(this, window);
+}
+
 void WindowTreeHostMus::StackAtTop() {
   delegate_->OnWindowTreeHostStackAtTop(this);
 }