Move RTCI420Frame with minor style changes.

webrtc/api/objc/RTCI420Frame+Private.h

Index: webrtc/api/objc/RTCI420Frame+Private.h
diff --git a/webrtc/base/objc/NSString+StdString.h b/webrtc/api/objc/RTCI420Frame+Private.h
similarity index 70%
copy from webrtc/base/objc/NSString+StdString.h
copy to webrtc/api/objc/RTCI420Frame+Private.h
index 532032b3fca66977290d612f1e998c205cceef00..b28717e2d356aee869177c43266c0e53ff13d2a4 100644
--- a/webrtc/base/objc/NSString+StdString.h
+++ b/webrtc/api/objc/RTCI420Frame+Private.h
@@ -8,17 +8,16 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#import <Foundation/Foundation.h>
+#import "RTCI420Frame.h"
 
-#include <string>
+#include "talk/media/base/videoframe.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
-@interface NSString (StdString)
+@interface RTCI420Frame ()
 
-@property(nonatomic, readonly) std::string stdString;
-
-+ (std::string)stdStringForString:(NSString *)nsString;
+- (instancetype)initWithVideoFrame:(const cricket::VideoFrame *)videoFrame
+    NS_DESIGNATED_INITIALIZER;
 
 @end