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

net/cert/multi_log_ct_verifier_unittest.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/cert/multi_log_ct_verifier.h"
 
 #include <string>
 
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
@@ skipping 158 matching lines @@
     BoundNetLog bound_net_log =
       BoundNetLog::Make(&net_log, NetLog::SOURCE_CONNECT_JOB);
     return (VerifySinglePrecertificateChain(chain, bound_net_log, &result) &&
             CheckForSingleVerifiedSCTInResult(result) &&
             CheckForSCTOrigin(result,
                               ct::SignedCertificateTimestamp::SCT_EMBEDDED) &&
             CheckForEmbeddedSCTInNetLog(net_log));
   }
 
   // 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));
 
     if (histogram == NULL)
       return 0;
 
     scoped_ptr<base::HistogramSamples> samples = histogram->SnapshotSamples();
     return samples->GetCount(sample_index);
   }
 
@@ skipping 139 matching lines @@
   Mock::VerifyAndClearExpectations(&observer);
 
   EXPECT_CALL(observer, OnSCTVerified(embedded_sct_chain_.get(), _)).Times(0);
   verifier_->SetObserver(nullptr);
   ASSERT_TRUE(VerifySinglePrecertificateChain(embedded_sct_chain_));
 }
 
 }  // namespace
 
 }  // namespace net