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

services/ui/public/interfaces/window_tree.mojom

 // 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.
 
 module ui.mojom;
 
 import "cc/ipc/surface_id.mojom";
 import "services/ui/public/interfaces/cursor.mojom";
 import "services/ui/public/interfaces/event_matcher.mojom";
 import "services/ui/public/interfaces/mus_constants.mojom";
@@ skipping 270 matching lines @@
 
   // Called by the client to start a drag operation. |source_window_id| is the
   // source window, |drag_data| is the entire set of mime to raw data
   // mapping. We send this during the start of the drag because most views
   // clients will try to read all this data on first entry.
   PerformDragDrop(uint32 change_id,
                   uint32 source_window_id,
                   int32 drag_pointer,
                   map<string, array<uint8>> drag_data,
                   uint32 drag_operation);
+
+  // Called by the client to cancel any in progress drag drop operation. This
+  // will result in a change completed for the underlying change.
+  CancelDragDrop(uint32 window_id);
 };
 
 // Changes to windows are not sent to the connection that originated the
 // change. For example, if connection 1 changes the bounds of a window by
 // calling SetWindowBounds(), connection 1 does not receive
 // OnWindowBoundsChanged().
 interface WindowTreeClient {
   // Invoked when the client application has been embedded at |root|.
   // See Embed() on WindowTree for more details. |tree| will be a handle back to
   // the window manager service, unless the connection is to the root connection
@@ skipping 171 matching lines @@
   // See description of WindowManager for details.
   GetWindowManager(associated WindowManager& internal);
 };
 
 // Mus provides this interface as a way for clients to connect and obtain a
 // WindowTree handle with a supplied WindowTreeClient handle. The
 // WindowTreeClient has no roots, use NewTopLevelWindow() to create one.
 interface WindowTreeFactory {
   CreateWindowTree(WindowTree& tree_request, WindowTreeClient client);
 };