Add support for persistent sparse histograms.

base/metrics/sample_map.h

Index: base/metrics/sample_map.h
diff --git a/base/metrics/sample_map.h b/base/metrics/sample_map.h
index da536e31e676d1e71516caf5c03c535719168451..36d422a1f3dd7e9820665a78ea57593dc390341b 100644
--- a/base/metrics/sample_map.h
+++ b/base/metrics/sample_map.h
@@ -26,7 +26,7 @@ class BASE_EXPORT SampleMap : public HistogramSamples {
   explicit SampleMap(uint64_t id);
   ~SampleMap() override;
 
-  // HistogramSamples implementation:
+  // HistogramSamples:
   void Accumulate(HistogramBase::Sample value,
                   HistogramBase::Count count) override;
   HistogramBase::Count GetCount(HistogramBase::Sample value) const override;
@@ -34,9 +34,8 @@ class BASE_EXPORT SampleMap : public HistogramSamples {
   scoped_ptr<SampleCountIterator> Iterator() const override;
 
  protected:
-  bool AddSubtractImpl(
-      SampleCountIterator* iter,
-      HistogramSamples::Operator op) override;  // |op| is ADD or SUBTRACT.
+  // Performs arithemetic. |op| is ADD or SUBTRACT.
+  bool AddSubtractImpl(SampleCountIterator* iter, Operator op) override;
 
  private:
   std::map<HistogramBase::Sample, HistogramBase::Count> sample_counts_;
@@ -44,28 +43,6 @@ class BASE_EXPORT SampleMap : public HistogramSamples {
   DISALLOW_COPY_AND_ASSIGN(SampleMap);
 };
 
-class BASE_EXPORT SampleMapIterator : public SampleCountIterator {
- public:
-  typedef std::map<HistogramBase::Sample, HistogramBase::Count>
-      SampleToCountMap;
-
-  explicit SampleMapIterator(const SampleToCountMap& sample_counts);
-  ~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_