message: Make PropagteGroupId process duplicate child nodes only once.
diff: @@ -145,6 +145,11 @@ void EventNode::PropagateGroupId(int64 group_id) {
  group_id_ = group_id;
  SetGroupId(*visitor_, group_id, event_);
  for (const auto& child : children_) {
  + // Skip if it already belongs to a group. Some nodes may be added multiple
  + // times as child (e.g., sometimes async ops are executed synchronously and
  + // their nodes are added as child both in ConnectIntraThread and
  + // ConnectInterThread).
  + if (child->GetGroupId()) continue;
  child->PropagateGroupId(*group_id_);
  }
  }
  