Cleanup: Pass std::string as const reference if possible

chromeos/network/network_connection_handler.cc

Index: chromeos/network/network_connection_handler.cc
diff --git a/chromeos/network/network_connection_handler.cc b/chromeos/network/network_connection_handler.cc
index e8c49396ba43b295dd1b3b19259b8d0cb5c92a66..af187ee01ceb86b660624fa9a270b88936bf2c75 100644
--- a/chromeos/network/network_connection_handler.cc
+++ b/chromeos/network/network_connection_handler.cc
@@ -656,7 +656,7 @@ void NetworkConnectionHandler::HandleShillConnectFailure(
 }
 
 void NetworkConnectionHandler::CheckPendingRequest(
-    const std::string service_path) {
+    const std::string& service_path) {
   ConnectRequest* request = GetPendingRequest(service_path);
   DCHECK(request);
   if (request->connect_state == ConnectRequest::CONNECT_REQUESTED)