Expose TurnCustomizer as a native ObjC property.

Bug: webrtc:8471
Change-Id: Ic0d904b0c3742e552f20fbebb26d55d2bdd76c72
Reviewed-on: https://webrtc-review.googlesource.com/16566
Commit-Queue: Zeke Chin <tkchin@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20481}

sdk/objc/Framework/Classes/PeerConnection/RTCConfiguration+Native.h

@@ -0,0 +1,27 @@
+/*
+ *  Copyright 2017 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/RTCConfiguration.h"
+
+#include "api/peerconnectioninterface.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface RTCConfiguration ()
+
+/** Optional TurnCustomizer.
+ *  With this class one can modify outgoing TURN messages.
+ *  The object passed in must remain valid until PeerConnection::Close() is called.
+ */
+@property(nonatomic, nullable) webrtc::TurnCustomizer *turnCustomizer;
+
+@end
+
+NS_ASSUME_NONNULL_END