Expose webSocketDidOpen in RCTReconnectingWebSocket

Summary: Expose webSocketDidOpen in RCTReconnectingWebSocket in order to get notified when the reconnecting websocket is opened to the endpoint.

Reviewed By: emilsjolander

Differential Revision: D5725547

fbshipit-source-id: e904c5a84d670ecf936993ec1739614f99fce09c

Libraries/WebSocket/RCTWebSocketObserver.m

@@ -66,6 +66,10 @@
   }
 }
 
+- (void)webSocketDidOpen:(RCTSRWebSocket *)webSocket
+{
+}
+
 - (void)webSocket:(RCTSRWebSocket *)webSocket didCloseWithCode:(NSInteger)code reason:(NSString *)reason wasClean:(BOOL)wasClean
 {
 }