Change uses of base::JoinString to pass StringPieces where possible.

extensions/browser/api/networking_private/networking_cast_private_delegate.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "extensions/browser/api/networking_private/networking_cast_private_delegate.h"
 
 #include <utility>
 
 #include "base/strings/string_util.h"
 
 namespace extensions {
 
 NetworkingCastPrivateDelegate::Credentials::Credentials(
     const std::string& certificate,
     const std::vector<std::string>& intermediate_certificates,
     const std::string& signed_data,
     const std::string& device_ssid,
     const std::string& device_serial,
     const std::string& device_bssid,
     const std::string& public_key,
     const std::string& nonce)
     : certificate_(certificate),
       intermediate_certificates_(intermediate_certificates),
       signed_data_(signed_data),
       device_bssid_(device_bssid),
       public_key_(public_key) {
-  std::vector<std::string> data_parts(
-      {device_ssid, device_serial, device_bssid, public_key, nonce});
-  unsigned_data_ = base::JoinString(data_parts, ",");
+  unsigned_data_ = base::JoinString(
+      {device_ssid, device_serial, device_bssid, public_key, nonce}, ",");
 }
 
 NetworkingCastPrivateDelegate::Credentials::~Credentials() {}
 
 }  // namespace extensions