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

chromeos/dbus/fake_shill_manager_client.cc

Index: chromeos/dbus/fake_shill_manager_client.cc
diff --git a/chromeos/dbus/fake_shill_manager_client.cc b/chromeos/dbus/fake_shill_manager_client.cc
index 337041b9e489b511827dbd9e8bce3648f02dcdef..c44f7cf8be0840b1392003871a73f2587d14255f 100644
--- a/chromeos/dbus/fake_shill_manager_client.cc
+++ b/chromeos/dbus/fake_shill_manager_client.cc
@@ -1115,8 +1115,9 @@ bool FakeShillManagerClient::ParseOption(const std::string& arg0,
   return SetInitialNetworkState(arg0, arg1);
 }
 
-bool FakeShillManagerClient::SetInitialNetworkState(std::string type_arg,
-                                                    std::string state_arg) {
+bool FakeShillManagerClient::SetInitialNetworkState(
+    const std::string& type_arg,
+    const std::string& state_arg) {
   int state_arg_as_int = -1;
   base::StringToInt(state_arg, &state_arg_as_int);