Refactor of Histogram related code

net/disk_cache/stats_histogram.h

Index: net/disk_cache/stats_histogram.h
===================================================================
--- net/disk_cache/stats_histogram.h	(revision 148055)
+++ net/disk_cache/stats_histogram.h	(working copy)
@@ -29,9 +29,11 @@
     }
   };
 
-  StatsHistogram(const std::string& name, Sample minimum,
-                 Sample maximum, size_t bucket_count)
-      : Histogram(name, minimum, maximum, bucket_count), init_(false) {}
+  StatsHistogram(const std::string& name,
+                 Sample minimum,
+                 Sample maximum,
+                 size_t bucket_count)
+      : Histogram(name, minimum, maximum, bucket_count, NULL), init_(false) {}
   virtual ~StatsHistogram();
 
   static StatsHistogram* FactoryGet(const std::string& name);
@@ -44,7 +46,6 @@
   virtual void SnapshotSample(SampleSet* sample) const OVERRIDE;
   virtual Inconsistencies FindCorruption(
       const SampleSet& snapshot) const OVERRIDE;
-  virtual uint32 CalculateRangeChecksum() const OVERRIDE;
 
  private:
   bool init_;