Enable iOS AppRTCDemo send side BWE.

webrtc/base/objc/RTCFieldTrials.mm

+/*
+ *  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 "webrtc/base/objc/RTCFieldTrials.h"
+
+#include <memory>
+#include "webrtc/system_wrappers/include/field_trial_default.h"
+
+#import <Foundation/Foundation.h>
+#import "webrtc/base/objc/RTCLogging.h"
+
+static NSString * const kRTCEnableSendSideBweString =
+    @"WebRTC-SendSideBwe/Enabled/";
+static std::unique_ptr<char[]> gFieldTrialInitString;
+
+void RTCInitFieldTrials(RTCFieldTrialOptions options) {
+  NSMutableString *fieldTrialInitString = [NSMutableString string];
+  if (options & RTCFieldTrialOptionsSendSideBwe) {
+    [fieldTrialInitString appendString:kRTCEnableSendSideBweString];
+  }
+  size_t len = fieldTrialInitString.length + 1;
+  gFieldTrialInitString.reset(new char[len]);
+  if (![fieldTrialInitString getCString:gFieldTrialInitString.get()
+                              maxLength:len
+                               encoding:NSUTF8StringEncoding]) {
+    RTCLogError(@"Failed to convert field trial string.");
+    return;
+  }
+  webrtc::field_trial::InitFieldTrialsFromString(gFieldTrialInitString.get());
+}