Make configuration logic harsher in FlexfecReceiveStream.

webrtc/call/flexfec_receive_stream.h

 /*
  *  Copyright (c) 2016 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.
  */
 
@@ skipping 24 matching lines @@
 
   // Implements webrtc::FlexfecReceiveStream.
   void Start() override;
   void Stop() override;
   Stats GetStats() const override;
 
  private:
   rtc::CriticalSection crit_;
   bool started_ GUARDED_BY(crit_);
 
-  Config config_;
+  const Config config_;
   const std::unique_ptr<FlexfecReceiver> receiver_;
 };
 
 }  // namespace internal
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_CALL_FLEXFEC_RECEIVE_STREAM_H_