Compile rtc_api_objc on Mac.

webrtc/api/objc/RTCMediaConstraints.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
 
 @interface RTCMediaConstraints : NSObject
 
-- (instancetype)init NS_UNAVAILABLE;
+- (nonnull instancetype)init NS_UNAVAILABLE;
 
 /** Initialize with mandatory and/or optional constraints. */
-- (instancetype)initWithMandatoryConstraints:
-    (nullable NSDictionary<NSString *, NSString *> *)mandatory
+- (nonnull instancetype)initWithMandatoryConstraints:
+    (nullable NSDictionary/*<NSString *, NSString *>*/ *)mandatory
                          optionalConstraints:
-    (nullable NSDictionary<NSString *, NSString *> *)optional
+    (nullable NSDictionary/*<NSString *, NSString *>*/ *)optional
     NS_DESIGNATED_INITIALIZER;
 
 @end
 
 NS_ASSUME_NONNULL_END