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

services/ui/public/interfaces/window_tree.mojom

Index: services/ui/public/interfaces/window_tree.mojom
diff --git a/services/ui/public/interfaces/window_tree.mojom b/services/ui/public/interfaces/window_tree.mojom
index 51c1726c17a6b0d02e096c822f26836c7b746310..d2343dbf791bcff7713eaaa2ac630d8b6d30557a 100644
--- a/services/ui/public/interfaces/window_tree.mojom
+++ b/services/ui/public/interfaces/window_tree.mojom
@@ -264,6 +264,10 @@ interface WindowTree {
   // window manager change the focus to the next activatable window.
   DeactivateWindow(uint32 window_id);
 
+  // Stacks the window |above_id| above |below_id|. These two windows must
+  // share the same parent.
+  StackAbove(uint32 change_id, uint32 above_id, uint32 below_id);
+
   // Stacks the window above all sibling windows.
   StackAtTop(uint32 change_id, uint32 window_id);