mus drag and drop: Add API to let the client cancel the drag.

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

Index: services/ui/public/cpp/tests/test_window_tree.cc
diff --git a/services/ui/public/cpp/tests/test_window_tree.cc b/services/ui/public/cpp/tests/test_window_tree.cc
index 888e8ae53fc5c220ace8c0473bf2417bc6fc14de..ace3f89d7cad713db026b5394ecb11b87f3ca3ea 100644
--- a/services/ui/public/cpp/tests/test_window_tree.cc
+++ b/services/ui/public/cpp/tests/test_window_tree.cc
@@ -178,6 +178,8 @@ void TestWindowTree::PerformDragDrop(
     mojo::Map<mojo::String, mojo::Array<uint8_t>> drag_data,
     uint32_t drag_operation) {}
 
+void TestWindowTree::CancelDragDrop(uint32_t window_id) {}
+
 void TestWindowTree::PerformWindowMove(uint32_t change_id,
                                        uint32_t window_id,
                                        mojom::MoveLoopSource source,