Add SampleMap and use it in SparseHistogram

base/metrics/sample_vector.h

Index: base/metrics/sample_vector.h
diff --git a/base/metrics/sample_vector.h b/base/metrics/sample_vector.h
index 8938d362a754b99626b7110d7b6dcc540f3913bb..5eecc7ffd6393e9e11310d1a15a3fbb35021f994 100644
--- a/base/metrics/sample_vector.h
+++ b/base/metrics/sample_vector.h
@@ -58,6 +58,7 @@ class BASE_EXPORT_PRIVATE SampleVectorIterator : public SampleCountIterator {
  public:
   SampleVectorIterator(const std::vector<HistogramBase::Count>* counts,
                        const BucketRanges* bucket_ranges);
+  virtual ~SampleVectorIterator();
 
   // SampleCountIterator implementation:
   virtual bool Done() const OVERRIDE;
@@ -65,6 +66,8 @@ class BASE_EXPORT_PRIVATE SampleVectorIterator : public SampleCountIterator {
   virtual void Get(HistogramBase::Sample* min,
                    HistogramBase::Sample* max,
                    HistogramBase::Count* count) const OVERRIDE;
+
+  // SampleVector uses predefined buckets, so iterator can return bucket index.
   virtual bool GetBucketIndex(size_t* index) const OVERRIDE;
 
  private: