Enable iOS AppRTCDemo send side BWE.

webrtc/base/objc/RTCFieldTrials.h

 /*
  *  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 <Foundation/Foundation.h>
+#include "webrtc/base/objc/RTCMacros.h"
 
-#import "webrtc/base/objc/RTCMacros.h"
+typedef NS_OPTIONS(NSUInteger, RTCFieldTrialOptions) {
+  RTCFieldTrialOptionsNone = 0,
+  RTCFieldTrialOptionsSendSideBwe = 1 << 0,
+};
 
-RTC_EXPORT void RTCSetupInternalTracer();
-/** Starts capture to specified file. Must be a valid writable path.
- *  Returns YES if capture starts.
+/** Must be called before any other call into WebRTC. See:
+ *  webrtc/system_wrappers/include/field_trial_default.h
  */
-RTC_EXPORT BOOL RTCStartInternalCapture(NSString *filePath);
-RTC_EXPORT void RTCStopInternalCapture();
-RTC_EXPORT void RTCShutdownInternalTracer();
+RTC_EXPORT void RTCInitFieldTrials(RTCFieldTrialOptions options);