Move RTCI420Frame with minor style changes.

webrtc/api/objc/RTCVideoFrame+Private.h

Index: webrtc/api/objc/RTCVideoFrame+Private.h
diff --git a/webrtc/base/objc/NSString+StdString.h b/webrtc/api/objc/RTCVideoFrame+Private.h
similarity index 64%
copy from webrtc/base/objc/NSString+StdString.h
copy to webrtc/api/objc/RTCVideoFrame+Private.h
index 8bf6cc94be0346e398ab1a980af6378f06ccb7a5..954344aee1b9c2095f03277bc1cc4a800e35e1a9 100644
--- a/webrtc/base/objc/NSString+StdString.h
+++ b/webrtc/api/objc/RTCVideoFrame+Private.h
@@ -8,18 +8,16 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#import <Foundation/Foundation.h>
+#import "RTCVideoFrame.h"
 
-#include <string>
+#include "talk/media/base/videoframe.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
-@interface NSString (StdString)
+@interface RTCVideoFrame ()
 
-@property(nonatomic, readonly) std::string stdString;
-
-+ (std::string)stdStringForString:(NSString *)nsString;
-+ (NSString *)stringForStdString:(const std::string&)stdString;
+- (instancetype)initWithNativeFrame:(const cricket::VideoFrame *)nativeFrame
+    NS_DESIGNATED_INITIALIZER;
 
 @end