Reorganize histograms for persistence.

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_
 
 #include <map>
 
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_samples.h"
 
 namespace base {
 
 class BASE_EXPORT SampleMap : public HistogramSamples {
  public:
   SampleMap();
+  explicit SampleMap(uint64_t id);
   ~SampleMap() override;
 
   // HistogramSamples implementation:
   void Accumulate(HistogramBase::Sample value,
                   HistogramBase::Count count) override;
   HistogramBase::Count GetCount(HistogramBase::Sample value) const override;
   HistogramBase::Count TotalCount() const override;
   scoped_ptr<SampleCountIterator> Iterator() const override;
 
  protected:
@@ skipping 25 matching lines @@
  private:
   void SkipEmptyBuckets();
 
   SampleToCountMap::const_iterator iter_;
   const SampleToCountMap::const_iterator end_;
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_SAMPLE_MAP_H_