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

extensions/browser/api/networking_private/networking_cast_private_delegate.cc

Index: extensions/browser/api/networking_private/networking_cast_private_delegate.cc
diff --git a/extensions/browser/api/networking_private/networking_cast_private_delegate.cc b/extensions/browser/api/networking_private/networking_cast_private_delegate.cc
index b290223dec2185c33d90d3bac993cbed9ae6ea69..de3a19b8719a8a206625b898d36c1726dc162b91 100644
--- a/extensions/browser/api/networking_private/networking_cast_private_delegate.cc
+++ b/extensions/browser/api/networking_private/networking_cast_private_delegate.cc
@@ -24,9 +24,8 @@ NetworkingCastPrivateDelegate::Credentials::Credentials(
       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() {}