dcsctp: Add OnBufferedAmountLow in Send Queue

This adds the necessary properties and callback to the Send Queue to
support the bufferedAmount & bufferedAmountLowThreshold properties and
the bufferedamountlow event in RTCDataChannel.

The public API changes and socket support comes in a follow-up CL.

Bug: webrtc:12794
Change-Id: I12a16f44f775da3711f3aa52a68a0bf24f70d2f8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/219690
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34142}

net/dcsctp/socket/dcsctp_socket.cc

@@ -167,7 +167,9 @@ DcSctpSocket::DcSctpSocket(absl::string_view log_prefix,
           TimerOptions(options.t2_shutdown_timeout,
                        TimerBackoffAlgorithm::kExponential,
                        options.max_retransmissions))),
-      send_queue_(log_prefix_, options_.max_send_buffer_size) {}
+      send_queue_(log_prefix_,
+                  options_.max_send_buffer_size,
+                  [](StreamID stream_id) {}) {}
 
 std::string DcSctpSocket::log_prefix() const {
   return log_prefix_ + "[" + std::string(ToString(state_)) + "] ";