iOS: Add MedianSlopeFilter field trial.

webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm

 /*
  *  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 "WebRTC/RTCFieldTrials.h"
 
 #include <memory>
 
 #import "WebRTC/RTCLogging.h"
 
 #include "webrtc/system_wrappers/include/field_trial_default.h"
 
 NSString * const kRTCFieldTrialAudioSendSideBweKey = @"WebRTC-Audio-SendSideBwe";
 NSString * const kRTCFieldTrialFlexFec03Key = @"WebRTC-FlexFEC-03";
 NSString * const kRTCFieldTrialImprovedBitrateEstimateKey = @"WebRTC-ImprovedBitrateEstimate";
+NSString * const kRTCFieldTrialMedianSlopeFilterKey = @"WebRTC-BweMedianSlopeFilter";
 NSString * const kRTCFieldTrialTrendlineFilterKey = @"WebRTC-BweTrendlineFilter";
 NSString * const kRTCFieldTrialH264HighProfileKey = @"WebRTC-H264HighProfile";
 NSString * const kRTCFieldTrialEnabledValue = @"Enabled";
 
 static std::unique_ptr<char[]> gFieldTrialInitString;
 
+NSString *RTCFieldTrialMedianSlopeFilterValue(
+    size_t windowSize, double thresholdGain) {
+  NSString *format = @"Enabled-%zu,%lf";
+  return [NSString stringWithFormat:format, windowSize, thresholdGain];
+}
+
 NSString *RTCFieldTrialTrendlineFilterValue(
     size_t windowSize, double smoothingCoeff, double thresholdGain) {
   NSString *format = @"Enabled-%zu,%lf,%lf";
   return [NSString stringWithFormat:format, windowSize, smoothingCoeff, thresholdGain];
 }
 
 void RTCInitFieldTrialDictionary(NSDictionary<NSString *, NSString *> *fieldTrials) {
   if (!fieldTrials) {
     RTCLogWarning(@"No fieldTrials provided.");
     return;
   }
   // Assemble the keys and values into the field trial string.
   // We don't perform any extra format checking. That should be done by the underlying WebRTC calls.
   NSMutableString *fieldTrialInitString = [NSMutableString string];
   for (NSString *key in fieldTrials) {
     NSString *fieldTrialEntry = [NSString stringWithFormat:@"%@/%@/", key, fieldTrials[key]];
     [fieldTrialInitString appendString:fieldTrialEntry];
   }
   size_t len = fieldTrialInitString.length + 1;
   gFieldTrialInitString.reset(new char[len]);
   if (![fieldTrialInitString getCString:gFieldTrialInitString.get()
                               maxLength:len
                                encoding:NSUTF8StringEncoding]) {
     RTCLogError(@"Failed to convert field trial string.");
     return;
   }
   webrtc::field_trial::InitFieldTrialsFromString(gFieldTrialInitString.get());
 }