iOS AppRTCMobile: Close peerconnection when disconnecting

We currently don't close the peerconnection before deallocing. That
could potentially cause race conditions if it's still being processed on
other threads.

BUG=webrtc:7976

Review-Url: https://codereview.webrtc.org/2976983002
Cr-Commit-Position: refs/heads/master@{#19121}

webrtc/examples/objc/AppRTCMobile/ARDAppClient.m

@@ -304,6 +304,7 @@ static int const kKbpsMultiplier = 1000;
   [_factory stopAecDump];
   [_peerConnection stopRtcEventLog];
 #endif
+  [_peerConnection close];
   _peerConnection = nil;
   self.state = kARDAppClientStateDisconnected;
 #if defined(WEBRTC_IOS)