Summary:
Idle WebSocket connections get reset after a few minutes of inactivity. To prevent this, most WebSocket implementations offer sending special ping messages. This PR adds a method `sendPing()` to WebSocket. Ping payloads are not supported.
Manual testing can be done by adding `connection.on('ping', _ => console.log('Received ping'));` to a ws connection or using a packet sniffer while sending pings.
Closes https://github.com/facebook/react-native/pull/8505
Differential Revision: D3516260
Pulled By: dmmiller
fbshipit-source-id: cfebf5899188ae53254d5be6b666a9075e0eed89
4ac4f86bf5