StreamingLevels are not marked pending kill causes missing necessarily object references cleanup

1. Open the [Link Removed]
2. PIE and press “F” key, a streaming leve is loaded.
3. Exit PIE

A log is attached.[Link Removed], the issue is from the CL#19387283.

This sample project isn't the perfect example to show the issue, because it could be avoid by using weakPtr to solve it, but licensee said it could reference any kind of UObject in practice, so we'd better to make sure all the streaming levels are marked pending kill as well.