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..4d69dfef876fa43a77593214415e55ccb706d0d0 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_;
@@ -52,7 +51,7 @@ class BASE_EXPORT SampleMapIterator : public SampleCountIterator {
   explicit SampleMapIterator(const SampleToCountMap& sample_counts);
   ~SampleMapIterator() override;
 
-  // SampleCountIterator implementation:
+  // SampleCountIterator:
   bool Done() const override;
   void Next() override;
   void Get(HistogramBase::Sample* min,