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

net/cert/internal/parse_certificate_unittest.cc

Index: net/cert/internal/parse_certificate_unittest.cc
diff --git a/net/cert/internal/parse_certificate_unittest.cc b/net/cert/internal/parse_certificate_unittest.cc
index 7089d458e04169ddbed9c2d69dd5a108c3c99867..cf6da4afc82bb57b8240c2b15a135fbbc0507fbe 100644
--- a/net/cert/internal/parse_certificate_unittest.cc
+++ b/net/cert/internal/parse_certificate_unittest.cc
@@ -22,7 +22,7 @@ std::string ToString(const der::GeneralizedTime& time) {
       time.month, time.day, time.hours, time.minutes, time.seconds);
 }
 
-std::string GetFilePath(const std::string file_name) {
+std::string GetFilePath(const std::string& file_name) {
   return std::string("net/data/parse_certificate_unittest/") + file_name;
 }