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

net/cert/multi_log_ct_verifier_unittest.cc

Index: net/cert/multi_log_ct_verifier_unittest.cc
diff --git a/net/cert/multi_log_ct_verifier_unittest.cc b/net/cert/multi_log_ct_verifier_unittest.cc
index a994eb218f25131163a439493a446e0b159c98be..492c21a06d496921d48025bd1f5e940db6004b6a 100644
--- a/net/cert/multi_log_ct_verifier_unittest.cc
+++ b/net/cert/multi_log_ct_verifier_unittest.cc
@@ -176,7 +176,8 @@ class MultiLogCTVerifierTest : public ::testing::Test {
   }
 
   // Histogram-related helper methods
-  int GetValueFromHistogram(std::string histogram_name, int sample_index) {
+  int GetValueFromHistogram(const std::string& histogram_name,
+                            int sample_index) {
     base::Histogram* histogram = static_cast<base::Histogram*>(
         base::StatisticsRecorder::FindHistogram(histogram_name));