mus: Implement interwindow drag and drop

services/ui/ws/window_server.cc

Index: services/ui/ws/window_server.cc
diff --git a/services/ui/ws/window_server.cc b/services/ui/ws/window_server.cc
index 5e1128644a673db5bd75d6d322fe4afd7e0a6fbb..169dec71f01ba692d4f3b48e777bf7b1dc44f5d8 100644
--- a/services/ui/ws/window_server.cc
+++ b/services/ui/ws/window_server.cc
@@ -484,6 +484,18 @@ gfx::Rect WindowServer::GetCurrentMoveLoopRevertBounds() {
   return gfx::Rect();
 }
 
+void WindowServer::StartDragLoop(uint32_t change_id) {
+  current_drag_loop_ = change_id;
+}
+
+void WindowServer::EndDragLoop() {
+  current_drag_loop_.reset();
+}
+
+uint32_t WindowServer::GetCurrentDragLoopChangeId() {
+  return current_drag_loop_.value_or(0u);
+}
+
 bool WindowServer::GetAndClearInFlightWindowManagerChange(
     uint32_t window_manager_change_id,
     InFlightWindowManagerChange* change) {