Allow mocking SendSideCongestionController for Call tests.

webrtc/call/rtp_transport_controller_send.cc

 /*
  *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/call/rtp_transport_controller_send.h"
 
 namespace webrtc {
 
 RtpTransportControllerSend::RtpTransportControllerSend(
     Clock* clock,
     webrtc::RtcEventLog* event_log)
     : send_side_cc_(clock, nullptr /* observer */, event_log, &packet_router_) {
 }
 
-void RtpTransportControllerSend::RegisterNetworkObserver(
-    SendSideCongestionController::Observer* observer) {
-  // Must be called only once.
-  send_side_cc_.RegisterNetworkObserver(observer);
-}
-
 }  // namespace webrtc