Update from https://crrev.com/320343

base/test/histogram_tester.cc

Index: base/test/histogram_tester.cc
diff --git a/base/test/histogram_tester.cc b/base/test/histogram_tester.cc
index 412059a416e953f0d1edf90b8ca0011cd8939ef6..ea738b0557b6e864a7e68c7d840c079d5c5967d8 100644
--- a/base/test/histogram_tester.cc
+++ b/base/test/histogram_tester.cc
@@ -90,7 +90,7 @@ 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);
@@ -104,9 +104,10 @@ void HistogramTester::CheckBucketCount(
       << ").";
 }
 
-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);