aura-mus: add a drag representation image.

ui/aura/mus/window_tree_client.h

Index: ui/aura/mus/window_tree_client.h
diff --git a/ui/aura/mus/window_tree_client.h b/ui/aura/mus/window_tree_client.h
index 8c9c8098638f0f7289346dafd4ca188715984d42..350fc9d1b8615d9a443cad3c2d99e1d88e454914 100644
--- a/ui/aura/mus/window_tree_client.h
+++ b/ui/aura/mus/window_tree_client.h
@@ -421,6 +421,13 @@ class AURA_EXPORT WindowTreeClient
           transport_properties) override;
   void WmClientJankinessChanged(ClientSpecificId client_id,
                                 bool janky) override;
+  void WmBuildDragImage(const gfx::Point& cursor_location,
+                        const SkBitmap& drag_image,
+                        const gfx::Vector2d& drag_image_offset,
+                        ui::mojom::DragEventSource source) override;
+  void WmMoveDragImage(const gfx::Point& cursor_location,
+                       const WmMoveDragImageCallback& callback) override;
+  void WmDestroyDragImage() override;
   void WmPerformMoveLoop(uint32_t change_id,
                          Id window_id,
                          ui::mojom::MoveLoopSource source,