Compile rtc_api_objc on Mac.

webrtc/api/objc/RTCVideoFrame.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 <Foundation/Foundation.h>
 
 NS_ASSUME_NONNULL_BEGIN
+// TODO(hjon): Update nullability types. See http://crbug/webrtc/5592
 
 @interface RTCVideoFrame : NSObject
 
 /** Width without rotation applied. */
 @property(nonatomic, readonly) size_t width;
 
 /** Height without rotation applied. */
 @property(nonatomic, readonly) size_t height;
 @property(nonatomic, readonly) size_t chromaWidth;
 @property(nonatomic, readonly) size_t chromaHeight;
 @property(nonatomic, readonly) size_t chromaSize;
 // These can return NULL if the object is not backed by a buffer.
 @property(nonatomic, readonly, nullable) const uint8_t *yPlane;
 @property(nonatomic, readonly, nullable) const uint8_t *uPlane;
 @property(nonatomic, readonly, nullable) const uint8_t *vPlane;
 @property(nonatomic, readonly) int32_t yPitch;
 @property(nonatomic, readonly) int32_t uPitch;
 @property(nonatomic, readonly) int32_t vPitch;
 
-- (instancetype)init NS_UNAVAILABLE;
+- (nonnull instancetype)init NS_UNAVAILABLE;
 
 @end
 
 NS_ASSUME_NONNULL_END