Move RTCI420Frame to webrtc/api/objc/RTCVideoFrame with minor style changes.

BUG=
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1533193003 .

Patch from Jon Hjelle <hjon@andyet.net>.

Cr-Commit-Position: refs/heads/master@{#11203}
diff --git a/webrtc/api/objc/RTCVideoFrame+Private.h b/webrtc/api/objc/RTCVideoFrame+Private.h
new file mode 100644
index 0000000..954344a
--- /dev/null
+++ b/webrtc/api/objc/RTCVideoFrame+Private.h
@@ -0,0 +1,24 @@
+/*
+ *  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 "RTCVideoFrame.h"
+
+#include "talk/media/base/videoframe.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface RTCVideoFrame ()
+
+- (instancetype)initWithNativeFrame:(const cricket::VideoFrame *)nativeFrame
+    NS_DESIGNATED_INITIALIZER;
+
+@end
+
+NS_ASSUME_NONNULL_END