net: replace usage of internal stream state with public api

Refs: #445

PR-URL: #34885
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>

lib/net.js

@@ -807,19 +807,18 @@ protoGetter('_bytesDispatched', function _bytesDispatched() {
 
 protoGetter('bytesWritten', function bytesWritten() {
   let bytes = this._bytesDispatched;
-  const state = this._writableState;
   const data = this._pendingData;
   const encoding = this._pendingEncoding;
+  const writableBuffer = this.writableBuffer;
 
-  if (!state)
+  if (!writableBuffer)
     return undefined;
 
-  this.writableBuffer.forEach(function(el) {
-    if (el.chunk instanceof Buffer)
-      bytes += el.chunk.length;
-    else
-      bytes += Buffer.byteLength(el.chunk, el.encoding);
-  });
+  for (const el of writableBuffer) {
+    bytes += el.chunk instanceof Buffer ?
+      el.chunk.length :
+      Buffer.byteLength(el.chunk, el.encoding);
+  }
 
   if (ArrayIsArray(data)) {
     // Was a writev, iterate over chunks to get total length