aura-mus: Implement stacking in DesktopWindowTreeHostMus.

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 ca5d986aad42bab1b419329eb18fe411a93aa131..c5283e81e309d0994c2e21c02db383b1149ce0c7 100644
--- a/ui/aura/mus/window_tree_host_mus.cc
+++ b/ui/aura/mus/window_tree_host_mus.cc
@@ -126,6 +126,10 @@ void WindowTreeHostMus::DeactivateWindow() {
   delegate_->OnWindowTreeHostDeactivateWindow(this);
 }
 
+void WindowTreeHostMus::StackAtTop() {
+  delegate_->OnWindowTreeHostStackAtTop(this);
+}
+
 display::Display WindowTreeHostMus::GetDisplay() const {
   display::Display display;
   display::Screen::GetScreen()->GetDisplayWithDisplayId(display_id_, &display);