iOS: Add MedianSlopeFilter field trial.

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

Index: webrtc/sdk/objc/Framework/Headers/WebRTC/RTCFieldTrials.h
diff --git a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCFieldTrials.h b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCFieldTrials.h
index 60c51d8a2be6716d7d5796dff15567afe6c48ed7..4ea9113688304f1a181e5191b29b723b132fd669 100644
--- a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCFieldTrials.h
+++ b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCFieldTrials.h
@@ -21,6 +21,11 @@ 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. */