Compile rtc_api_objc on Mac.

webrtc/api/objc/RTCIceServer.h

Index: webrtc/api/objc/RTCIceServer.h
diff --git a/webrtc/api/objc/RTCIceServer.h b/webrtc/api/objc/RTCIceServer.h
index be4e0d7b6e89da90cbbbc38faa53826bd3bc59b6..3a6d28ab8641bda0864593a985126b2b691296a5 100644
--- a/webrtc/api/objc/RTCIceServer.h
+++ b/webrtc/api/objc/RTCIceServer.h
@@ -15,7 +15,7 @@ NS_ASSUME_NONNULL_BEGIN
 @interface RTCIceServer : NSObject
 
 /** URI(s) for this server represented as NSStrings. */
-@property(nonatomic, copy, readonly) NSArray<NSString *> *urlStrings;
+@property(nonatomic, copy, readonly, nonnull) NSArray/*<NSString *>*/ *urlStrings;
 
 /** Username to use if this RTCIceServer object is a TURN server. */
 @property(nonatomic, copy, readonly, nullable) NSString *username;
@@ -23,16 +23,16 @@ NS_ASSUME_NONNULL_BEGIN
 /** Credential to use if this RTCIceServer object is a TURN server. */
 @property(nonatomic, copy, readonly, nullable) NSString *credential;
 
-- (instancetype)init NS_UNAVAILABLE;
+- (nonnull instancetype)init NS_UNAVAILABLE;
 
 /** Convenience initializer for a server with no authentication (e.g. STUN). */
-- (instancetype)initWithURLStrings:(NSArray<NSString *> *)urlStrings;
+- (nonnull instancetype)initWithURLStrings:(nonnull NSArray/*<NSString *>*/ *)urlStrings;
 
 /**
  * Initialize an RTCIceServer with its associated URLs, optional username,
  * optional credential, and credentialType.
  */
-- (instancetype)initWithURLStrings:(NSArray<NSString *> *)urlStrings
+- (nonnull instancetype)initWithURLStrings:(nonnull NSArray/*<NSString *>*/ *)urlStrings
                           username:(nullable NSString *)username
                         credential:(nullable NSString *)credential
     NS_DESIGNATED_INITIALIZER;