Improve object array chunking test in ParNew

JDK-8027545 is an RFE to enhance the object array chunking test in the oop closures for G1, to effectively make them similar to Parallel GC.

ParNew could also benefit from this optimization:

bool ParScanThreadState::should_be_partially_scanned(oop new_obj, oop old_obj) const {
  return new_obj->is_objArray() &&
         arrayOop(new_obj)->length() > ParGCArrayScanChunk &&
         new_obj != old_obj;
}