mash: Use the new mus drag and drop API to get drag working in mash.

services/ui/public/cpp/tests/test_window_tree.h

Index: services/ui/public/cpp/tests/test_window_tree.h
diff --git a/services/ui/public/cpp/tests/test_window_tree.h b/services/ui/public/cpp/tests/test_window_tree.h
index a8c24301f880189615b4f588c1eed24f3c6b9f6d..7deed9dfd7c4874136f30b2cb13ded2635828c10 100644
--- a/services/ui/public/cpp/tests/test_window_tree.h
+++ b/services/ui/public/cpp/tests/test_window_tree.h
@@ -105,7 +105,6 @@ class TestWindowTree : public mojom::WindowTree {
       override;
   void PerformDragDrop(uint32_t change_id,
                        uint32_t source_window_id,
-                       int32_t drag_pointer,
                        mojo::Map<mojo::String, mojo::Array<uint8_t>> drag_data,
                        uint32_t drag_operation) override;
   void CancelDragDrop(uint32_t window_id) override;