iOS: Add AudioSendSideBwe 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 2c538cdb46d5c5b5104f6d072a7d663c6f702581..ac3a4b3e48abeb53ed9a96de49c44ff3075a8e78 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
@@ -17,8 +17,9 @@
 #include "webrtc/system_wrappers/include/field_trial_default.h"
 
 static NSString * const kRTCEnableImprovedBitrateEstimateString =
-   @"WebRTC-ImprovedBitrateEstimate/Enabled/";
-
+    @"WebRTC-ImprovedBitrateEstimate/Enabled/";
+static NSString * const kRTCEnableAudioSendSideBweString =
+    @"WebRTC-Audio-SendSideBwe/Enabled/";
 static std::unique_ptr<char[]> gFieldTrialInitString;
 
 void RTCInitFieldTrials(RTCFieldTrialOptions options) {
@@ -26,6 +27,9 @@ void RTCInitFieldTrials(RTCFieldTrialOptions options) {
   if (options & RTCFieldTrialOptionsImprovedBitrateEstimate) {
     [fieldTrialInitString appendString:kRTCEnableImprovedBitrateEstimateString];
   }
+  if (options & RTCFieldTrialOptionsAudioSendSideBwe) {
+    [fieldTrialInitString appendString:kRTCEnableAudioSendSideBweString];
+  }
   size_t len = fieldTrialInitString.length + 1;
   gFieldTrialInitString.reset(new char[len]);
   if (![fieldTrialInitString getCString:gFieldTrialInitString.get()