Update API for Objective-C RTCAudioTrack and RTCVideoTrack.

webrtc/api/objc/RTCAudioTrack.mm

+/*
+ *  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 "RTCAudioTrack.h"
+
+#import "webrtc/api/objc/RTCAudioTrack+Private.h"
+#import "webrtc/api/objc/RTCMediaStreamTrack+Private.h"
+#import "webrtc/api/objc/RTCPeerConnectionFactory+Private.h"
+#import "webrtc/base/objc/NSString+StdString.h"
+
+@implementation RTCAudioTrack
+
+- (instancetype)initWithFactory:(RTCPeerConnectionFactory *)factory
+                        trackId:(NSString *)trackId {
+  NSParameterAssert(factory);
+  NSParameterAssert(trackId.length);
+  std::string nativeId = [NSString stdStringForString:trackId];
+  rtc::scoped_refptr<webrtc::AudioTrackInterface> track =
+      factory.nativeFactory->CreateAudioTrack(nativeId, nullptr);
+  return [self initWithNativeTrack:track type:RTCMediaStreamTrackTypeAudio];
+}
+
+- (instancetype)initWithNativeTrack:
+    (rtc::scoped_refptr<webrtc::MediaStreamTrackInterface>)nativeTrack
+                               type:(RTCMediaStreamTrackType)type {
+  NSParameterAssert(nativeTrack);
+  NSParameterAssert(type == RTCMediaStreamTrackTypeAudio);
+  return [super initWithNativeTrack:nativeTrack type:type];
+}
+
+#pragma mark - Private
+
+- (rtc::scoped_refptr<webrtc::AudioTrackInterface>)nativeAudioTrack {
+  return static_cast<webrtc::AudioTrackInterface *>(self.nativeTrack.get());
+}
+
+@end