Update API for Objective-C RTCAudioTrack and RTCVideoTrack.

webrtc/api/objc/RTCVideoTrack.h

+/*
+ *  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 "RTCMediaStreamTrack.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@protocol RTCVideoRenderer;
+@class RTCPeerConnectionFactory;
+@class RTCVideoSource;
+
+@interface RTCVideoTrack : RTCMediaStreamTrack
+
+/** The video source for this video track. */
+@property(nonatomic, readonly) RTCVideoSource *source;
+
+- (instancetype)init NS_UNAVAILABLE;
+
+/** Initialize an RTCVideoTrack with its source and an id. */
+- (instancetype)initWithFactory:(RTCPeerConnectionFactory *)factory
+                         source:(RTCVideoSource *)source
+                        trackId:(NSString *)trackId;
+
+/** Register a renderer that will render all frames received on this track. */
+- (void)addRenderer:(id<RTCVideoRenderer>)renderer;
+
+/** Deregister a renderer. */
+- (void)removeRenderer:(id<RTCVideoRenderer>)renderer;
+
+@end
+
+NS_ASSUME_NONNULL_END