Update API for Objective-C RTCMediaStream.

BUG=
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1558733002 .

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

Cr-Commit-Position: refs/heads/master@{#11351}
diff --git a/webrtc/api/objc/RTCMediaStream+Private.h b/webrtc/api/objc/RTCMediaStream+Private.h
new file mode 100644
index 0000000..2c2662b
--- /dev/null
+++ b/webrtc/api/objc/RTCMediaStream+Private.h
@@ -0,0 +1,32 @@
+/*
+ *  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 "RTCMediaStream.h"
+
+#include "talk/app/webrtc/mediastreaminterface.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface RTCMediaStream ()
+
+/**
+ * MediaStreamInterface representation of this RTCMediaStream object. This is
+ * needed to pass to the underlying C++ APIs.
+ */
+@property(nonatomic, readonly)
+    rtc::scoped_refptr<webrtc::MediaStreamInterface> nativeMediaStream;
+
+/** Initialize an RTCMediaStream from a native MediaStreamInterface. */
+- (instancetype)initWithNativeMediaStream:
+    (rtc::scoped_refptr<webrtc::MediaStreamInterface>)nativeMediaStream;
+
+@end
+
+NS_ASSUME_NONNULL_END