Restore type attributes and remove extraneous nullability annotations for Objective-C Mac build

BUG=webrtc:5592
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1773743002 .

Patch from Jon Hjelle <hjon@andyet.net>.

Cr-Commit-Position: refs/heads/master@{#11922}
diff --git a/webrtc/api/objc/RTCMediaStream.h b/webrtc/api/objc/RTCMediaStream.h
index 7df1ea8..e3ab754 100644
--- a/webrtc/api/objc/RTCMediaStream.h
+++ b/webrtc/api/objc/RTCMediaStream.h
@@ -11,7 +11,6 @@
 #import <Foundation/Foundation.h>
 
 NS_ASSUME_NONNULL_BEGIN
-// TODO(hjon): Update nullability types. See http://crbug/webrtc/5592
 
 @class RTCAudioTrack;
 @class RTCPeerConnectionFactory;
@@ -20,12 +19,10 @@
 @interface RTCMediaStream : NSObject
 
 /** The audio tracks in this stream. */
-@property(nonatomic, strong, readonly) NSArray *audioTracks;
-// @property(nonatomic, strong, readonly) NSArray<RTCAudioTrack *> *audioTracks;
+@property(nonatomic, strong, readonly) NSArray<RTCAudioTrack *> *audioTracks;
 
 /** The video tracks in this stream. */
-@property(nonatomic, strong, readonly) NSArray *videoTracks;
-// @property(nonatomic, strong, readonly) NSArray<RTCVideoTrack *> *videoTracks;
+@property(nonatomic, strong, readonly) NSArray<RTCVideoTrack *> *videoTracks;
 
 /** An identifier for this media stream. */
 @property(nonatomic, readonly) NSString *streamId;