Add SampleMap and use it in SparseHistogram

base/metrics/histogram_samples.h

Index: base/metrics/histogram_samples.h
diff --git a/base/metrics/histogram_samples.h b/base/metrics/histogram_samples.h
index 778b11debdc379c7c5bc410917c6011ec67ab896..c55b58442c5771d345128cc1f0a7a46e7744a0ab 100644
--- a/base/metrics/histogram_samples.h
+++ b/base/metrics/histogram_samples.h
@@ -42,11 +42,9 @@ class BASE_EXPORT HistogramSamples {
   HistogramBase::Count redundant_count() const { return redundant_count_; }
 
  protected:
-  // Based on |instruction| type, add or subtract sample counts data from the
-  // iterator.
-  enum Instruction { ADD, SUBTRACT };
-  virtual bool AddSubtractImpl(SampleCountIterator* iter,
-                               Instruction instruction) = 0;
+  // Based on |op| type, add or subtract sample counts data from the iterator.
+  enum Operator { ADD, SUBTRACT };
+  virtual bool AddSubtractImpl(SampleCountIterator* iter, Operator op) = 0;
 
   void IncreaseSum(int64 diff);
   void IncreaseRedundantCount(HistogramBase::Count diff);