message: remove empty line first then construct XPlaneBuilder to make  sure lines_by_id is updated.
diff: @@ -219,9 +219,9 @@ void NormalizeTimeLine(XSpace* space, uint64 start_time_ns) {
  }
  void MergePlanes(const XPlane& src_plane, XPlane* dst_plane) {
  + RemoveEmptyLines(dst_plane);
  XPlaneVisitor src(&src_plane);
  XPlaneBuilder dst(dst_plane);
  - RemoveEmptyLines(dst_plane);
  src.ForEachStat([&](const tensorflow::profiler::XStatVisitor& stat) {
  XStatMetadata* stat_metadata = dst.GetOrCreateStatMetadata(stat.Name());
  XStat* new_stat = dst.FindOrAddMutableStat(stat_metadata->id());
  