Revert "Remove Platform check from WebSocket module"

Summary:
This reverts commit b9be289.

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Fix facebook#18696

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Run Jest tests.
Run WebSocket example from RNTester on Android.

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[ANDROID][BUGFIX][WebSocketModule] - revert change that regresses WebSocketModule
Closes facebook#18733

Differential Revision: D7548850

Pulled By: hramos

fbshipit-source-id: b8c79810c1cd6e5a30ec4118bd5ff8ad719f04b9

Libraries/WebSocket/WebSocket.js

@@ -14,6 +14,7 @@ const EventTarget = require('event-target-shim');
 const NativeEventEmitter = require('NativeEventEmitter');
 const BlobManager = require('BlobManager');
 const NativeModules = require('NativeModules');
+const Platform = require('Platform');
 const WebSocketEvent = require('WebSocketEvent');
 
 /* $FlowFixMe(>=0.54.0 site=react_native_oss) This comment suppresses an error
@@ -205,7 +206,7 @@ class WebSocket extends EventTarget(...WEBSOCKET_EVENTS) {
   }
 
   _close(code?: number, reason?: string): void {
-    if (WebSocketModule.close.length === 3) {
+    if (Platform.OS === 'android') {
       // See https://developer.mozilla.org/en-US/docs/Web/API/CloseEvent
       const statusCode = typeof code === 'number' ? code : CLOSE_NORMAL;
       const closeReason = typeof reason === 'string' ? reason : '';