Compile rtc_api_objc on Mac.

webrtc/api/objc/RTCMediaStream.h

Index: webrtc/api/objc/RTCMediaStream.h
diff --git a/webrtc/api/objc/RTCMediaStream.h b/webrtc/api/objc/RTCMediaStream.h
index c63f9ed44337a2c4a1001df522b74d04dd781e0e..b3143604340af9a3141642369bb9c828489fdd92 100644
--- a/webrtc/api/objc/RTCMediaStream.h
+++ b/webrtc/api/objc/RTCMediaStream.h
@@ -11,6 +11,7 @@
 #import <Foundation/Foundation.h>
 
 NS_ASSUME_NONNULL_BEGIN
+// TODO(hjon): Update nullability types. See http://crbug/webrtc/5592
 
 @class RTCAudioTrack;
 @class RTCVideoTrack;
@@ -18,10 +19,12 @@ NS_ASSUME_NONNULL_BEGIN
 @interface RTCMediaStream : NSObject
 
 /** The audio tracks in this stream. */
-@property(nonatomic, strong, readonly) NSArray<RTCAudioTrack *> *audioTracks;
+@property(nonatomic, strong, readonly) NSArray *audioTracks;
+// @property(nonatomic, strong, readonly) NSArray<RTCAudioTrack *> *audioTracks;
 
 /** The video tracks in this stream. */
-@property(nonatomic, strong, readonly) NSArray<RTCVideoTrack *> *videoTracks;
+@property(nonatomic, strong, readonly) NSArray *videoTracks;
+// @property(nonatomic, strong, readonly) NSArray<RTCVideoTrack *> *videoTracks;
 
 /** An identifier for this media stream. */
 @property(nonatomic, readonly) NSString *streamId;