message: Prepare for usrsctp being updated.
  The signature of send_cb was changed, adding ulp_info. This change makes
  it easier to retrieve the SctpTransport pointer from the callback.
  Cr-Commit-Position: refs/heads/master@{#32067}
diff: @@ -393,6 +393,10 @@ class SctpTransport::UsrSctpWrapper {
  struct sctp_rcvinfo rcv,
  int flags,
  void* ulp_info) {
  + // Sanity check that both methods of getting the SctpTransport pointer
  + // yield the same result.
  + RTC_CHECK_EQ(static_cast<SctpTransport*>(ulp_info),
  + GetTransportFromSocket(sock));
  SctpTransport* transport = static_cast<SctpTransport*>(ulp_info);
  int result =
  transport->OnDataOrNotificationFromSctp(data, length, rcv, flags);
  @@ -426,6 +430,8 @@ class SctpTransport::UsrSctpWrapper {
  return transport;
  }
  + // TODO(crbug.com/webrtc/11899): This is a legacy callback signature, remove
  + // when usrsctp is updated.
  static int SendThresholdCallback(struct socket* sock, uint32_t sb_free) {
  // Fired on our I/O thread. SctpTransport::OnPacketReceived() gets
  // a packet containing acknowledgments, which goes into usrsctp_conninput,
  @@ -440,6 +446,21 @@ class SctpTransport::UsrSctpWrapper {
  transport->OnSendThresholdCallback();
  return 0;
  }
  +
  + static int SendThresholdCallback(struct socket* sock,
  + uint32_t sb_free,
  + void* ulp_info) {
  + // Sanity check that both methods of getting the SctpTransport pointer
  + // yield the same result.
  + RTC_CHECK_EQ(static_cast<SctpTransport*>(ulp_info),
  + GetTransportFromSocket(sock));
  + // Fired on our I/O thread. SctpTransport::OnPacketReceived() gets
  + // a packet containing acknowledgments, which goes into usrsctp_conninput,
  + // and then back here.
  + SctpTransport* transport = static_cast<SctpTransport*>(ulp_info);
  + transport->OnSendThresholdCallback();
  + return 0;
  + }
  };
  SctpTransport::SctpTransport(rtc::Thread* network_thread,
  