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

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "services/ui/public/cpp/tests/test_window_tree.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace ui {
 
@@ skipping 160 matching lines @@
   callback.Run(mojo::ScopedSharedBufferHandle());
 }
 
 void TestWindowTree::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) {}
 
+void TestWindowTree::CancelDragDrop(uint32_t window_id) {}
+
 void TestWindowTree::PerformWindowMove(uint32_t change_id,
                                        uint32_t window_id,
                                        mojom::MoveLoopSource source,
                                        const gfx::Point& cursor_location) {}
 
 void TestWindowTree::CancelWindowMove(uint32_t window_id) {}
 
 }  // namespace ui