Add a GN target for unit tests, get them working again and added a test.

BUG=webrtc:3417

Review-Url: https://codereview.webrtc.org/2050153003
Cr-Commit-Position: refs/heads/master@{#14959}
diff --git a/webrtc/examples/objc/AppRTCMobile/ARDAppClient.m b/webrtc/examples/objc/AppRTCMobile/ARDAppClient.m
index 9ef46b4..4a073f3 100644
--- a/webrtc/examples/objc/AppRTCMobile/ARDAppClient.m
+++ b/webrtc/examples/objc/AppRTCMobile/ARDAppClient.m
@@ -318,7 +318,9 @@
   _peerConnection = nil;
   self.state = kARDAppClientStateDisconnected;
 #if defined(WEBRTC_IOS)
-  RTCStopInternalCapture();
+  if (kARDAppClientEnableTracing) {
+    RTCStopInternalCapture();
+  }
 #endif
 }