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

ui/views/mus/desktop_window_tree_host_mus.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/mus/desktop_window_tree_host_mus.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "ui/aura/client/aura_constants.h"
 #include "ui/aura/client/cursor_client.h"
@@ skipping 388 matching lines @@
 }
 
 void DesktopWindowTreeHostMus::SetSize(const gfx::Size& size) {
   // Use GetBounds() as the origin of window() is always at 0, 0.
   gfx::Rect screen_bounds =
       gfx::ConvertRectToDIP(GetScaleFactor(), GetBoundsInPixels());
   screen_bounds.set_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() {
   // Request to the server to stack our current mus window at the top. Our
   // window() is a root, and we can't reach up past it so we can't just request
   // a Reorder(), which is what we'd do to reorder our own subwindows.
   WindowTreeHostMus::StackAtTop();
 }
 
 void DesktopWindowTreeHostMus::CenterWindow(const gfx::Size& size) {
@@ skipping 307 matching lines @@
     aura::client::FocusClient* focus_client,
     aura::Window* window) {
   if (window == this->window()) {
     desktop_native_widget_aura_->HandleActivationChanged(true);
   } else if (is_active_) {
     desktop_native_widget_aura_->HandleActivationChanged(false);
   }
 }
 
 }  // namespace views