Update API for Objective-C RTCAudioTrack and RTCVideoTrack.

webrtc/api/objc/RTCAudioTrack.h

Index: webrtc/api/objc/RTCAudioTrack.h
diff --git a/webrtc/base/objc/NSString+StdString.h b/webrtc/api/objc/RTCAudioTrack.h
similarity index 62%
copy from webrtc/base/objc/NSString+StdString.h
copy to webrtc/api/objc/RTCAudioTrack.h
index 8bf6cc94be0346e398ab1a980af6378f06ccb7a5..40474684d1f16d241e313a59cfa36fcf423b2577 100644
--- a/webrtc/base/objc/NSString+StdString.h
+++ b/webrtc/api/objc/RTCAudioTrack.h
@@ -8,18 +8,17 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#import <Foundation/Foundation.h>
-
-#include <string>
+#import "RTCMediaStreamTrack.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
-@interface NSString (StdString)
+@interface RTCAudioTrack : RTCMediaStreamTrack
 
-@property(nonatomic, readonly) std::string stdString;
+- (instancetype)init NS_UNAVAILABLE;
 
-+ (std::string)stdStringForString:(NSString *)nsString;
-+ (NSString *)stringForStdString:(const std::string&)stdString;
+/** Initialize an RTCAudioTrack with an id. */
+- (instancetype)initWithFactory:(RTCPeerConnectionFactory *)factory
+                        trackId:(NSString *)trackId;
 
 @end