Smooth BWE and pass it to Audio Network Adaptor.

webrtc/base/smoothing_filter.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.
  */
 
 #ifndef WEBRTC_BASE_SMOOTHING_FILTER_H_
 #define WEBRTC_BASE_SMOOTHING_FILTER_H_
 
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/base/exp_filter.h"
 #include "webrtc/base/optional.h"
 #include "webrtc/system_wrappers/include/clock.h"
 
 namespace webrtc {
 
 class SmoothingFilter {
  public:
   virtual ~SmoothingFilter() = default;
   virtual void AddSample(float sample) = 0;
   virtual rtc::Optional<float> GetAverage() const = 0;
+  virtual void SetTimeConstantMs(int time_constant_ms) = 0;
 };
 
 // SmoothingFilterImpl applies an exponential filter
 //   alpha = exp(-sample_interval / time_constant);
 //   y[t] = alpha * y[t-1] + (1 - alpha) * sample;
 class SmoothingFilterImpl final : public SmoothingFilter {
  public:
   // |time_constant_ms| is the time constant for the exponential filter.
   SmoothingFilterImpl(int time_constant_ms, const Clock* clock);
 
   void AddSample(float sample) override;
   rtc::Optional<float> GetAverage() const override;
+  void SetTimeConstantMs(int time_constant_ms) override;
 
  private:
-  const int time_constant_ms_;
+  int time_constant_ms_;
   const Clock* const clock_;
 
   bool first_sample_received_;
   bool initialized_;
   int64_t first_sample_time_ms_;
   int64_t last_sample_time_ms_;
   rtc::ExpFilter filter_;
 
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(SmoothingFilterImpl);
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_BASE_SMOOTHING_FILTER_H_