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

chromeos/network/shill_property_util.cc

Index: chromeos/network/shill_property_util.cc
diff --git a/chromeos/network/shill_property_util.cc b/chromeos/network/shill_property_util.cc
index 30a6925bb6af2f6696d18904a9c0b17186123bf1..c5cafc8395e4400a58c181cb5de11e08f51abc34 100644
--- a/chromeos/network/shill_property_util.cc
+++ b/chromeos/network/shill_property_util.cc
@@ -61,7 +61,7 @@ bool CopyStringFromDictionary(const base::DictionaryValue& source,
 
 }  // namespace
 
-void SetSSID(const std::string ssid, base::DictionaryValue* properties) {
+void SetSSID(const std::string& ssid, base::DictionaryValue* properties) {
   std::string hex_ssid = base::HexEncode(ssid.c_str(), ssid.size());
   properties->SetStringWithoutPathExpansion(shill::kWifiHexSsid, hex_ssid);
 }