Remove some uses of isdigit in net/, as it can be locale depedent.

net/ftp/ftp_network_transaction.cc

Index: net/ftp/ftp_network_transaction.cc
diff --git a/net/ftp/ftp_network_transaction.cc b/net/ftp/ftp_network_transaction.cc
index 018a892904ea54274ebd32d499da3c13bc916eaf..dbe1154f8262fc7260833bb7abdcf43aba9e8137 100644
--- a/net/ftp/ftp_network_transaction.cc
+++ b/net/ftp/ftp_network_transaction.cc
@@ -135,8 +135,8 @@ bool ExtractPortFromEPSVResponse(const FtpCtrlResponse& response, int* port) {
   char separator = epsv_line[start + 1];
 
   // Make sure we have "(<d><d><d>...", where <d> is not a number.
-  if (isdigit(separator) || epsv_line[start + 2] != separator ||
-      epsv_line[start + 3] != separator) {
+  if ((separator >= '0' && separator <= '9') ||
+      epsv_line[start + 2] != separator || epsv_line[start + 3] != separator) {
     return false;
   }