Revert of Move JoinString to the base namespace

chromeos/network/onc/onc_validator.cc

Index: chromeos/network/onc/onc_validator.cc
diff --git a/chromeos/network/onc/onc_validator.cc b/chromeos/network/onc/onc_validator.cc
index 4c2067a7bdb2d17914f64b33a808292fdde852f9..4e2e732073a7c6ddf72f555467019ed536aed846 100644
--- a/chromeos/network/onc/onc_validator.cc
+++ b/chromeos/network/onc/onc_validator.cc
@@ -325,7 +325,7 @@
                             const std::string& separator) {
   std::vector<std::string> string_vector;
   std::copy(strings.begin(), strings.end(), std::back_inserter(string_vector));
-  return base::JoinString(string_vector, separator);
+  return JoinString(string_vector, separator);
 }
 
 }  // namespace
@@ -952,7 +952,7 @@
 }
 
 std::string Validator::MessageHeader() {
-  std::string path = path_.empty() ? "toplevel" : base::JoinString(path_, ".");
+  std::string path = path_.empty() ? "toplevel" : JoinString(path_, ".");
   std::string message = "At " + path + ": ";
   return message;
 }