Update from https://crrev.com/333737

base/metrics/sample_map.h

 // Copyright (c) 2012 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.
 
 // SampleMap implements HistogramSamples interface. It is used by the
 // SparseHistogram class to store samples.
 
 #ifndef BASE_METRICS_SAMPLE_MAP_H_
 #define BASE_METRICS_SAMPLE_MAP_H_
 
@@ skipping 38 matching lines @@
   ~SampleMapIterator() override;
 
   // SampleCountIterator implementation:
   bool Done() const override;
   void Next() override;
   void Get(HistogramBase::Sample* min,
            HistogramBase::Sample* max,
            HistogramBase::Count* count) const override;
 
  private:
+  void SkipEmptyBuckets();
+
   SampleToCountMap::const_iterator iter_;
   const SampleToCountMap::const_iterator end_;
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_SAMPLE_MAP_H_