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

webrtc/api/objc/RTCMediaStream.mm

Index: webrtc/api/objc/RTCMediaStream.mm
diff --git a/webrtc/api/objc/RTCMediaStream.mm b/webrtc/api/objc/RTCMediaStream.mm
index 3c3e4d906455cedbc4e343d086df6903007f12a2..666022301a11ec65ffc4232ddaf55fea1f7e076a 100644
--- a/webrtc/api/objc/RTCMediaStream.mm
+++ b/webrtc/api/objc/RTCMediaStream.mm
@@ -19,8 +19,6 @@
 #import "webrtc/api/objc/RTCVideoTrack+Private.h"
 #import "webrtc/base/objc/NSString+StdString.h"
 
-// TODO(hjon): Update nullability types. See http://crbug/webrtc/5592
-
 @implementation RTCMediaStream {
   NSMutableArray *_audioTracks;
   NSMutableArray *_videoTracks;
@@ -37,13 +35,11 @@
   return [self initWithNativeMediaStream:stream];
 }
 
-- (NSArray *)audioTracks {
-// - (NSArray<RTCAudioTrack *> *)audioTracks {
+- (NSArray<RTCAudioTrack *> *)audioTracks {
   return [_audioTracks copy];
 }
 
-- (NSArray *)videoTracks {
-// - (NSArray<RTCVideoTrack *> *)videoTracks {
+- (NSArray<RTCVideoTrack *> *)videoTracks {
   return [_videoTracks copy];
 }