Update from https://crrev.com/320343

base/test/histogram_tester.h

 // 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.
 
 #ifndef BASE_TEST_HISTOGRAM_TESTER_H_
 #define BASE_TEST_HISTOGRAM_TESTER_H_
 
 #include <map>
 #include <string>
 
@@ skipping 41 matching lines @@
   scoped_ptr<HistogramSamples> GetHistogramSamplesSinceCreation(
       const std::string& histogram_name);
 
  private:
   // Verifies and asserts that value in the |sample| bucket matches the
   // |expected_count|. The bucket's current value is determined from |samples|
   // and is modified based on the snapshot stored for histogram |name|.
   void CheckBucketCount(const std::string& name,
                         base::HistogramBase::Sample sample,
                         base::Histogram::Count expected_count,
-                        base::HistogramSamples& samples) const;
+                        const base::HistogramSamples& samples) const;
 
   // Verifies that the total number of values recorded for the histogram |name|
   // is |expected_count|. This is checked against |samples| minus the snapshot
   // that was taken for |name|.
   void CheckTotalCount(const std::string& name,
                        base::Histogram::Count expected_count,
-                       base::HistogramSamples& samples) const;
+                       const base::HistogramSamples& samples) const;
 
   // Used to determine the histogram changes made during this instance's
   // lifecycle. This instance takes ownership of the samples, which are deleted
   // when the instance is destroyed.
   std::map<std::string, HistogramSamples*> histograms_snapshot_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramTester);
 };
 
 }  // namespace base
 
 #endif  // BASE_TEST_HISTOGRAM_TESTER_H_