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

services/ui/public/cpp/window.h

Index: services/ui/public/cpp/window.h
diff --git a/services/ui/public/cpp/window.h b/services/ui/public/cpp/window.h
index c781c37d600a05d13eb37fdbda21cad754383885..e33c3bde212e3f68658360fe9a770ebd605ec0ef 100644
--- a/services/ui/public/cpp/window.h
+++ b/services/ui/public/cpp/window.h
@@ -243,6 +243,9 @@ class Window {
       const SkBitmap& bitmap,
       const base::Callback<void(bool, uint32_t)>& callback);
 
+  // Cancels the in progress drag started with PerformDragDrop().
+  void CancelDragDrop();
+
   // Tells the window manager to take control of moving the window. Returns
   // true if the move wasn't canceled.
   void PerformWindowMove(mojom::MoveLoopSource source,