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

services/ui/ws/window_tree_client_unittest.cc

Index: services/ui/ws/window_tree_client_unittest.cc
diff --git a/services/ui/ws/window_tree_client_unittest.cc b/services/ui/ws/window_tree_client_unittest.cc
index e06fefb47b684fb081aec31c806294ee7404e31b..871d70a0f08c93c552aaba6ca2d83cec1fa254c5 100644
--- a/services/ui/ws/window_tree_client_unittest.cc
+++ b/services/ui/ws/window_tree_client_unittest.cc
@@ -476,6 +476,10 @@ class TestWindowTreeClient : public mojom::WindowTreeClient,
   }
   void WmCancelMoveLoop(uint32_t window_id) override { NOTIMPLEMENTED(); }
   void WmDeactivateWindow(uint32_t window_id) override { NOTIMPLEMENTED(); }
+  void WmStackAbove(uint32_t change_id, uint32_t above_id,
+                    uint32_t below_id) override {
+    NOTIMPLEMENTED();
+  }
   void WmStackAtTop(uint32_t change_id, uint32_t window_id) override {
     NOTIMPLEMENTED();
   }