mus: Implement interwindow drag and drop

services/ui/ws/window_manager_state.h

Index: services/ui/ws/window_manager_state.h
diff --git a/services/ui/ws/window_manager_state.h b/services/ui/ws/window_manager_state.h
index a0726788e2e2954d08e1f70e29a2d6a748e50255..7e5921eecec8d7ec09159d8db6666aa6213577a6 100644
--- a/services/ui/ws/window_manager_state.h
+++ b/services/ui/ws/window_manager_state.h
@@ -61,6 +61,13 @@ class WindowManagerState : public EventDispatcherDelegate {
   void ReleaseCaptureBlockedByModalWindow(const ServerWindow* modal_window);
   void ReleaseCaptureBlockedByAnyModalWindow();
 
+  void SetDragDropSourceWindow(
+      uint32_t change_id,
+      mojo::Map<mojo::String, mojo::Array<uint8_t>> drag_data,
+      uint32_t drag_operation,
+      WindowTree* window_tree,
+      ServerWindow* capture_window);
+
   void AddSystemModalWindow(ServerWindow* window);
 
   // TODO(sky): EventDispatcher is really an implementation detail and should