iOS: Add MedianSlopeFilter field trial.

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

Index: webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm b/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
index 89cf3c8ea041dc060130b16f2d0cf9890efadbb4..a3534964049275f4bf23ed1b4eed4e6ef12e7e87 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
@@ -19,12 +19,19 @@
 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";