iOS: Add MedianSlopeFilter field trial.

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCFieldTrials.h

 /*
  *  Copyright 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.
  */
 
 #import <Foundation/Foundation.h>
 
 #import <WebRTC/RTCMacros.h>
 
 /** The only valid value for the following if set is kRTCFieldTrialEnabledValue. */
 RTC_EXTERN NSString * const kRTCFieldTrialAudioSendSideBweKey;
 RTC_EXTERN NSString * const kRTCFieldTrialFlexFec03Key;
 RTC_EXTERN NSString * const kRTCFieldTrialImprovedBitrateEstimateKey;
 RTC_EXTERN NSString * const kRTCFieldTrialH264HighProfileKey;
 
 /** The valid value for field trials above. */
 RTC_EXTERN NSString * const kRTCFieldTrialEnabledValue;
 
+/** Use a string returned by RTCFieldTrialMedianSlopeFilterValue as the value. */
+RTC_EXTERN NSString * const kRTCFieldTrialMedianSlopeFilterKey;
+RTC_EXTERN NSString *RTCFieldTrialMedianSlopeFilterValue(
+    size_t windowSize, double thresholdGain);
+
 /** Use a string returned by RTCFieldTrialTrendlineFilterValue as the value. */
 RTC_EXTERN NSString * const kRTCFieldTrialTrendlineFilterKey;
 /** Returns a valid value for kRTCFieldTrialTrendlineFilterKey. */
 RTC_EXTERN NSString *RTCFieldTrialTrendlineFilterValue(
     size_t windowSize, double smoothingCoeff, double thresholdGain);
 
 /** Initialize field trials using a dictionary mapping field trial keys to their values. See above
  *  for valid keys and values.
  *  Must be called before any other call into WebRTC. See:
  *  webrtc/system_wrappers/include/field_trial_default.h
  */
 RTC_EXTERN void RTCInitFieldTrialDictionary(NSDictionary<NSString *, NSString *> *fieldTrials);