Enable iOS AppRTCDemo send side BWE.

webrtc/base/BUILD.gn

 # Copyright (c) 2014 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("//build/config/crypto.gni")
 import("//build/config/ui.gni")
@@ skipping 619 matching lines @@
     configs += [ "..:common_config" ]
     public_configs = [ "..:common_inherited_config" ]
 
     sources = [
       "objc/NSString+StdString.h",
       "objc/NSString+StdString.mm",
       "objc/RTCCameraPreviewView.h",
       "objc/RTCCameraPreviewView.m",
       "objc/RTCDispatcher.h",
       "objc/RTCDispatcher.m",
+      "objc/RTCFieldTrials.h",
+      "objc/RTCFieldTrials.mm",
       "objc/RTCFileLogger.h",
       "objc/RTCFileLogger.mm",
       "objc/RTCLogging.h",
       "objc/RTCLogging.mm",
       "objc/RTCMacros.h",
       "objc/RTCSSLAdapter.h",
       "objc/RTCSSLAdapter.mm",
       "objc/RTCTracing.h",
       "objc/RTCTracing.mm",
     ]
   }
 }
 
 source_set("gtest_prod") {
   sources = [
     "gtest_prod_util.h",
   ]
 }