mus drag and drop: return the completed effect to the caller.

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 6d4782528b725a5ea7beb1088809b7f37f3846d0..9a7c076dcf755e7f23ee4360a0fc68e1d150aaee 100644
--- a/services/ui/public/interfaces/window_tree.mojom
+++ b/services/ui/public/interfaces/window_tree.mojom
@@ -452,8 +452,12 @@ interface WindowTreeClient {
                  gfx.mojom.Point position,
                  uint32 effect_bitmask) => (uint32 effect_taken);
 
-  // Called after OnDragDrop completes for every connection which received an
-  // OnDragDropStart() message. This signals that a client can forget the
+  // Called on the client that requested PerformDragDrop() to return which drag
+  // action was completed.
+  OnDragCompletedReturnValue(uint32 window, uint32 effect_taken);
+
+  // Called after OnCompleteDrop completes for every connection which received
+  // an OnDragDropStart() message. This signals that a client can forget the
   // |drag_data| passed in via the first message.
   OnDragDropDone();