Add stats overlay to iOS AppRTCDemo.

BUG=
R=jiayl@webrtc.org

Review URL: https://codereview.webrtc.org/1289623005 .

Cr-Commit-Position: refs/heads/master@{#9714}
diff --git a/webrtc/examples/objc/AppRTCDemo/mac/APPRTCViewController.m b/webrtc/examples/objc/AppRTCDemo/mac/APPRTCViewController.m
index 96ad7c9..dec41ea 100644
--- a/webrtc/examples/objc/AppRTCDemo/mac/APPRTCViewController.m
+++ b/webrtc/examples/objc/AppRTCDemo/mac/APPRTCViewController.m
@@ -270,6 +270,10 @@
   [self disconnect];
 }
 
+- (void)appClient:(ARDAppClient *)client
+      didGetStats:(NSArray *)stats {
+}
+
 #pragma mark - APPRTCMainViewDelegate
 
 - (void)appRTCMainView:(APPRTCMainView*)mainView