Use sockets with unread data if they've never been used before.

net/socket/socks5_client_socket.h

Index: net/socket/socks5_client_socket.h
diff --git a/net/socket/socks5_client_socket.h b/net/socket/socks5_client_socket.h
index 45216244f108ad1b8c1b61bdde8e669532c15522..75f75310d72a8ba4097dda6825981ad64c218f30 100644
--- a/net/socket/socks5_client_socket.h
+++ b/net/socket/socks5_client_socket.h
@@ -99,6 +99,7 @@ class NET_EXPORT_PRIVATE SOCKS5ClientSocket : public StreamSocket {
 
   void DoCallback(int result);
   void OnIOComplete(int result);
+  void OnReadWriteComplete(const CompletionCallback& callback, int result);
 
   int DoLoop(int last_io_result);
   int DoHandshakeRead();
@@ -143,6 +144,8 @@ class NET_EXPORT_PRIVATE SOCKS5ClientSocket : public StreamSocket {
 
   size_t read_header_size;
 
+  bool was_ever_used_;
+
   HostResolver::RequestInfo host_request_info_;
 
   BoundNetLog net_log_;