Update API for Objective-C RTCPeerConnectionFactory.

BUG=
R=jiayl@webrtc.org, tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1558473002 .

Patch from Jon Hjelle <hjon@andyet.net>.

Cr-Commit-Position: refs/heads/master@{#11326}

webrtc/api/objc/RTCPeerConnectionFactory+Private.h

@@ -0,0 +1,30 @@
+/*
+ *  Copyright 2015 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 "RTCPeerConnectionFactory.h"
+
+#include "talk/app/webrtc/peerconnectionfactory.h"
+#include "webrtc/base/scoped_ref_ptr.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface RTCPeerConnectionFactory ()
+
+/**
+ * PeerConnectionFactoryInterface created and held by this
+ * RTCPeerConnectionFactory object. This is needed to pass to the underlying
+ * C++ APIs.
+ */
+@property(nonatomic, readonly)
+    rtc::scoped_refptr<webrtc::PeerConnectionFactoryInterface> nativeFactory;
+
+@end
+
+NS_ASSUME_NONNULL_END