iOS: Add FlexFEC-03 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 ac3a4b3e48abeb53ed9a96de49c44ff3075a8e78..3057a17aedf34fdf089ffc66198b802a2338e335 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
@@ -20,6 +20,8 @@ static NSString * const kRTCEnableImprovedBitrateEstimateString =
     @"WebRTC-ImprovedBitrateEstimate/Enabled/";
 static NSString * const kRTCEnableAudioSendSideBweString =
     @"WebRTC-Audio-SendSideBwe/Enabled/";
+static NSString * const kRTCEnableFlexFec03String =
+    @"WebRTC-FlexFEC-03/Enabled/";
 static std::unique_ptr<char[]> gFieldTrialInitString;
 
 void RTCInitFieldTrials(RTCFieldTrialOptions options) {
@@ -30,6 +32,9 @@ void RTCInitFieldTrials(RTCFieldTrialOptions options) {
   if (options & RTCFieldTrialOptionsAudioSendSideBwe) {
     [fieldTrialInitString appendString:kRTCEnableAudioSendSideBweString];
   }
+  if (options & RTCFieldTrialOptionsFlexFec03) {
+    [fieldTrialInitString appendString:kRTCEnableFlexFec03String];
+  }
   size_t len = fieldTrialInitString.length + 1;
   gFieldTrialInitString.reset(new char[len]);
   if (![fieldTrialInitString getCString:gFieldTrialInitString.get()