aura-mus: Implement stacking in DesktopWindowTreeHostMus.

services/ui/ws/window_tree.h

Index: services/ui/ws/window_tree.h
diff --git a/services/ui/ws/window_tree.h b/services/ui/ws/window_tree.h
index 36dba018cfb594186de239f59f43228aa5a40250..cf0e37959de8cffeea7227aeeaa07e63ed99f075 100644
--- a/services/ui/ws/window_tree.h
+++ b/services/ui/ws/window_tree.h
@@ -445,6 +445,7 @@ class WindowTree : public mojom::WindowTree,
   void SetCanAcceptDrops(Id window_id, bool accepts_drops) override;
   void SetHitTestMask(Id transport_window_id,
                       const base::Optional<gfx::Rect>& mask) override;
+  void StackAtTop(uint32_t change_id, Id window_id) override;
   void GetWindowManagerClient(
       mojo::AssociatedInterfaceRequest<mojom::WindowManagerClient> internal)
       override;
@@ -489,6 +490,8 @@ class WindowTree : public mojom::WindowTree,
   bool IsWindowKnownForAccessPolicy(const ServerWindow* window) const override;
   bool IsWindowRootOfAnotherTreeForAccessPolicy(
       const ServerWindow* window) const override;
+  bool IsWindowCreatedByWindowManager(
+      const ServerWindow* window) const override;
 
   // DragSource:
   void OnDragCompleted(bool success, uint32_t action_taken) override;