Compile rtc_api_objc on Mac.

webrtc/api/objc/RTCIceCandidate.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 RTCIceCandidate : NSObject
 
 /**
  * If present, the identifier of the "media stream identification" for the media
  * component this candidate is associated with.
  */
 @property(nonatomic, readonly, nullable) NSString *sdpMid;
 
 /**
  * The index (starting at zero) of the media description this candidate is
  * associated with in the SDP.
  */
 @property(nonatomic, readonly) NSInteger sdpMLineIndex;
 
 /** The SDP string for this candidate. */
-@property(nonatomic, readonly) NSString *sdp;
+@property(nonatomic, readonly, nonnull) NSString *sdp;
 
-- (instancetype)init NS_UNAVAILABLE;
+- (nonnull instancetype)init NS_UNAVAILABLE;
 
 /**
  * Initialize an RTCIceCandidate from SDP.
  */
-- (instancetype)initWithSdp:(NSString *)sdp
-              sdpMLineIndex:(NSInteger)sdpMLineIndex
-                     sdpMid:(nullable NSString *)sdpMid
+- (nonnull instancetype)initWithSdp:(nonnull NSString *)sdp
+                      sdpMLineIndex:(NSInteger)sdpMLineIndex
+                             sdpMid:(nullable NSString *)sdpMid
     NS_DESIGNATED_INITIALIZER;
 
 @end
 
 NS_ASSUME_NONNULL_END