Update from https://crrev.com/320343

base/test/histogram_tester.cc

 // Copyright 2014 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 "base/test/histogram_tester.h"
 
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/stl_util.h"
@@ skipping 72 matching lines @@
       histograms_snapshot_[histogram_name];
   if (named_original_samples)
     named_samples->Subtract(*named_original_samples);
   return named_samples.Pass();
 }
 
 void HistogramTester::CheckBucketCount(
     const std::string& name,
     base::HistogramBase::Sample sample,
     base::HistogramBase::Count expected_count,
-    base::HistogramSamples& samples) const {
+    const base::HistogramSamples& samples) const {
   int actual_count = samples.GetCount(sample);
   std::map<std::string, HistogramSamples*>::const_iterator histogram_data;
   histogram_data = histograms_snapshot_.find(name);
   if (histogram_data != histograms_snapshot_.end())
     actual_count -= histogram_data->second->GetCount(sample);
 
   EXPECT_EQ(expected_count, actual_count)
       << "Histogram \"" << name
       << "\" does not have the right number of samples (" << expected_count
       << ") in the expected bucket (" << sample << "). It has (" << actual_count
       << ").";
 }
 
-void HistogramTester::CheckTotalCount(const std::string& name,
-                                      base::HistogramBase::Count expected_count,
-                                      base::HistogramSamples& samples) const {
+void HistogramTester::CheckTotalCount(
+    const std::string& name,
+    base::HistogramBase::Count expected_count,
+    const base::HistogramSamples& samples) const {
   int actual_count = samples.TotalCount();
   std::map<std::string, HistogramSamples*>::const_iterator histogram_data;
   histogram_data = histograms_snapshot_.find(name);
   if (histogram_data != histograms_snapshot_.end())
     actual_count -= histogram_data->second->TotalCount();
 
   EXPECT_EQ(expected_count, actual_count)
       << "Histogram \"" << name
       << "\" does not have the right total number of samples ("
       << expected_count << "). It has (" << actual_count << ").";
 }
 
 }  // namespace base