Simplify QpackEncoderStreamSender API. Instead of each Send*() method returning the number of bytes encoded, return the total number of bytes written from Flush(). This is a follow-up to cr/273553497. gfe-relnote: n/a, change to QUIC v99-only code. Protected by existing disabled gfe2_reloadable_flag_quic_enable_version_99. PiperOrigin-RevId: 273775165 Change-Id: Ifaf1b55d6d0ab26cc3ab23686bcb7427e3c9627f