Tweaks for new Objective-C API.

BUG=

Review URL: https://codereview.webrtc.org/1696673003

Cr-Commit-Position: refs/heads/master@{#11872}
diff --git a/webrtc/api/objc/RTCPeerConnection.h b/webrtc/api/objc/RTCPeerConnection.h
index 86668a5..72645b5 100644
--- a/webrtc/api/objc/RTCPeerConnection.h
+++ b/webrtc/api/objc/RTCPeerConnection.h
@@ -181,7 +181,7 @@
  *  statistics are gathered for all tracks.
  */
 - (void)statsForTrack:
-    (nonnull RTCMediaStreamTrack *)mediaStreamTrack
+    (nullable RTCMediaStreamTrack *)mediaStreamTrack
      statsOutputLevel:(RTCStatsOutputLevel)statsOutputLevel
     completionHandler:
     (nullable void (^)(NSArray * _Nonnull stats))completionHandler;