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

services/ui/public/interfaces/window_tree.mojom

Index: services/ui/public/interfaces/window_tree.mojom
diff --git a/services/ui/public/interfaces/window_tree.mojom b/services/ui/public/interfaces/window_tree.mojom
index fa2023e7a39ae2d3bdbee5629fed6e3cf725e44e..de0b9c7cb0690fa0348ba6a2ee0424604c15c77f 100644
--- a/services/ui/public/interfaces/window_tree.mojom
+++ b/services/ui/public/interfaces/window_tree.mojom
@@ -288,6 +288,10 @@ interface WindowTree {
                   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