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

services/ui/ws/event_dispatcher.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/ws/event_dispatcher.h"
 
 #include <algorithm>
 
 #include "base/time/time.h"
 #include "services/ui/ws/accelerator.h"
@@ skipping 158 matching lines @@
     DragTargetConnection* source_connection,
     int32_t drag_pointer,
     mojo::Map<mojo::String, mojo::Array<uint8_t>> mime_data,
     uint32_t drag_operations) {
   CancelImplicitCaptureExcept(nullptr);
   drag_controller_ = base::MakeUnique<DragController>(
       drag_source, window, source_connection, drag_pointer,
       std::move(mime_data), drag_operations);
 }
 
+void EventDispatcher::CancelDragDrop() {
+  if (drag_controller_)
+    drag_controller_->Cancel();
+}
+
 void EventDispatcher::EndDragDrop() {
   drag_controller_.reset();
 }
 
 void EventDispatcher::OnWillDestroyDragTargetConnection(
     DragTargetConnection* connection) {
   if (drag_controller_)
     drag_controller_->OnWillDestroyDragTargetConnection(connection);
 }
 
@@ skipping 413 matching lines @@
 
 void EventDispatcher::OnWindowDestroyed(ServerWindow* window) {
   CancelPointerEventsToTarget(window);
 
   if (mouse_cursor_source_window_ == window)
     mouse_cursor_source_window_ = nullptr;
 }
 
 }  // namespace ws
 }  // namespace ui