aura-mus: create an interactive ui test for drag and drop.

services/ui/ws/window_server_test_impl.h

Index: services/ui/ws/window_server_test_impl.h
diff --git a/services/ui/ws/window_server_test_impl.h b/services/ui/ws/window_server_test_impl.h
index 9a6dc788ee4282d219a0b5073abd6834149cf1fd..17fa60beaf80b225c7da1fe30fe418ae7d71b6da 100644
--- a/services/ui/ws/window_server_test_impl.h
+++ b/services/ui/ws/window_server_test_impl.h
@@ -27,6 +27,9 @@ class WindowServerTestImpl : public mojom::WindowServerTest {
   void EnsureClientHasDrawnWindow(
       const std::string& client_name,
       const EnsureClientHasDrawnWindowCallback& callback) override;
+  void DispatchEvent(int64_t display_id,
+                     std::unique_ptr<ui::Event> event,
+                     const DispatchEventCallback& cb) override;
 
   WindowServer* window_server_;