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

services/ui/public/cpp/window_tree_client.cc

 // Copyright 2014 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/window_tree_client.h"
 
 #include <stddef.h>
 
 #include <string>
 #include <utility>
@@ skipping 656 matching lines @@
       base::MakeUnique<InFlightDragChange>(window, ChangeType::DRAG_LOOP));
   current_drag_state_.reset(new CurrentDragState{
       current_drag_change, ui::mojom::kDropEffectNone, callback});
 
   tree_->PerformDragDrop(
       current_drag_change, window->server_id(), drag_pointer,
       mojo::Map<mojo::String, mojo::Array<uint8_t>>::From(drag_data),
       drag_operation);
 }
 
+void WindowTreeClient::CancelDragDrop(Window* window) {
+  tree_->CancelDragDrop(window->server_id());

[sky, 2016/09/21 20:40:56]

Please add a comment that cleanup is done when server receives cancel.

+}
+
 void WindowTreeClient::PerformWindowMove(
     Window* window,
     ui::mojom::MoveLoopSource source,
     const gfx::Point& cursor_location,
     const base::Callback<void(bool)>& callback) {
   DCHECK(on_current_move_finished_.is_null());
   on_current_move_finished_ = callback;
 
   current_move_loop_change_ = ScheduleInFlightChange(
       base::MakeUnique<InFlightDragChange>(window, ChangeType::MOVE_LOOP));
@@ skipping 719 matching lines @@
     Window* window,
     const gfx::Vector2d& offset,
     const gfx::Insets& hit_area) {
   if (window_manager_internal_client_) {
     window_manager_internal_client_->SetUnderlaySurfaceOffsetAndExtendedHitArea(
         server_id(window), offset.x(), offset.y(), hit_area);
   }
 }
 
 }  // namespace ui