Add WritevData() in StreamDelegateInterface and let QuicSession::WritevData implement it.
gfe-relnote: no behavior change. not protected.
PiperOrigin-RevId: 295818988
Change-Id: Ic5f9e7bf8ec7c4fc63feaaee96b213911a386433
QUICHE (QUIC, Http/2, Etc) is Google‘s implementation of QUIC and related protocols. It powers Chromium as well as Google’s QUIC servers and some other projects.