mus drag and drop: return the completed effect to the caller.

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 9eff23e1f90d11cedccda23d14e8f20c04d80001..59bf4a2f9ba2e3d3432b9152c9b76b92599000f4 100644
--- a/services/ui/ws/window_tree_client_unittest.cc
+++ b/services/ui/ws/window_tree_client_unittest.cc
@@ -399,6 +399,12 @@ class TestWindowTreeClient : public mojom::WindowTreeClient,
                       const OnCompleteDropCallback& callback) override {
     NOTIMPLEMENTED();
   }
+
+  void OnDragCompletedReturnValue(uint32_t window,
+                                  uint32_t action_taken) override {
+    NOTIMPLEMENTED();
+  }
+
   void OnDragDropDone() override { NOTIMPLEMENTED(); }
 
   void OnChangeCompleted(uint32_t change_id, bool success) override {