Update API for Objective-C RTCDataChannel.

BUG=

Review URL: https://codereview.webrtc.org/1545393003

Cr-Commit-Position: refs/heads/master@{#11362}
diff --git a/webrtc/api/objc/RTCDataChannel+Private.h b/webrtc/api/objc/RTCDataChannel+Private.h
new file mode 100644
index 0000000..cc44923
--- /dev/null
+++ b/webrtc/api/objc/RTCDataChannel+Private.h
@@ -0,0 +1,49 @@
+/*
+ *  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 "RTCDataChannel.h"
+
+#include "talk/app/webrtc/datachannelinterface.h"
+#include "webrtc/base/scoped_ref_ptr.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface RTCDataBuffer ()
+
+/**
+ * The native DataBuffer representation of this RTCDatabuffer object. This is
+ * needed to pass to the underlying C++ APIs.
+ */
+@property(nonatomic, readonly) const webrtc::DataBuffer *nativeDataBuffer;
+
+/** Initialize an RTCDataBuffer from a native DataBuffer. */
+- (instancetype)initWithNativeBuffer:(const webrtc::DataBuffer&)nativeBuffer;
+
+@end
+
+
+@interface RTCDataChannel ()
+
+/** Initialize an RTCDataChannel from a native DataChannelInterface. */
+- (instancetype)initWithNativeDataChannel:
+    (rtc::scoped_refptr<webrtc::DataChannelInterface>)nativeDataChannel
+    NS_DESIGNATED_INITIALIZER;
+
++ (webrtc::DataChannelInterface::DataState)
+    nativeDataChannelStateForState:(RTCDataChannelState)state;
+
++ (RTCDataChannelState)dataChannelStateForNativeState:
+    (webrtc::DataChannelInterface::DataState)nativeState;
+
++ (NSString *)stringForState:(RTCDataChannelState)state;
+
+@end
+
+NS_ASSUME_NONNULL_END