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

services/ui/public/interfaces/window_manager.mojom

Index: services/ui/public/interfaces/window_manager.mojom
diff --git a/services/ui/public/interfaces/window_manager.mojom b/services/ui/public/interfaces/window_manager.mojom
index 14615fcdf8d0b2663af4c0dadd5fa4591c290193..0f404dac312836d0e2d1416cd41c7969d5bc2994 100644
--- a/services/ui/public/interfaces/window_manager.mojom
+++ b/services/ui/public/interfaces/window_manager.mojom
@@ -191,6 +191,9 @@ interface WindowManager {
   // to the next. The window manager may completely ignore this message.
   WmDeactivateWindow(uint32 window_id);
 
+  // Asks the WindowMangaer to stack |above_id| in front of |below_id|.
+  WmStackAbove(uint32 change_id, uint32 above_id, uint32 below_id);
+
   // Asks the WindowManager to stack |window_id| as the first child of its
   // window manager owned parent.
   WmStackAtTop(uint32 change_id, uint32 window_id);