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

services/ui/public/cpp/window_tree_client.h

Index: services/ui/public/cpp/window_tree_client.h
diff --git a/services/ui/public/cpp/window_tree_client.h b/services/ui/public/cpp/window_tree_client.h
index d6c9fbf2d3a9a9c353cdb58d60dd69a7f729a51c..643855bec19ed462049cb17fbfa130c612767fa7 100644
--- a/services/ui/public/cpp/window_tree_client.h
+++ b/services/ui/public/cpp/window_tree_client.h
@@ -185,6 +185,10 @@ class WindowTreeClient : public mojom::WindowTreeClient,
       const SkBitmap& bitmap,
       const base::Callback<void(bool, uint32_t)>& callback);
 
+  // Cancels a in progress drag drop. (If no drag is in progress, does
+  // nothing.)
+  void CancelDragDrop(Window* window);
+
   // Performs a window move. |callback| will be asynchronously called with the
   // whether the move loop completed successfully.
   void PerformWindowMove(Window* window,