Remove stream pointer from QuicSession::WritevData. The session only uses the pointer to access stream_bytes_written. It could simply be replaced by a |is_retransmission| boolean. This change makes the boundary of streams and sessions clearer. gfe-relnote: no behavior change, not protected. PiperOrigin-RevId: 296504335 Change-Id: I349bdadadd923e9dcb5b03231ce351e4f1a25c96