Update iOS AppRTCDemo to use the updated Objective-C API.

webrtc/api/objc/RTCPeerConnection.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.
  */
 
@@ skipping 163 matching lines @@
     configuration:(nonnull RTCDataChannelConfiguration *)configuration;
 
 @end
 
 @interface RTCPeerConnection (Stats)
 
 /** Gather stats for the given RTCMediaStreamTrack. If |mediaStreamTrack| is nil
  *  statistics are gathered for all tracks.
  */
 - (void)statsForTrack:
-    (nonnull RTCMediaStreamTrack *)mediaStreamTrack
+    (nullable RTCMediaStreamTrack *)mediaStreamTrack
      statsOutputLevel:(RTCStatsOutputLevel)statsOutputLevel
     completionHandler:
     (nullable void (^)(NSArray * _Nonnull stats))completionHandler;
     // (nullable void (^)(NSArray<RTCStatsReport *> *stats))completionHandler;
 
 @end
 
 NS_ASSUME_NONNULL_END