Refactor of Histogram related code

base/metrics/bucket_ranges.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // BucketRanges stores the vector of ranges that delimit what samples are
 // tallied in the corresponding buckets of a histogram. Histograms that have
 // same ranges for all their corresponding buckets should share the same
 // BucketRanges object.
 //
 // E.g. A 5 buckets LinearHistogram with 1 as minimal value and 4 as maximal
@@ skipping 23 matching lines @@
   ~BucketRanges();
 
   size_t size() const { return ranges_.size(); }
   HistogramBase::Sample range(size_t i) const { return ranges_[i]; }
   void set_range(size_t i, HistogramBase::Sample value);
   uint32 checksum() const { return checksum_; }
   void set_checksum(uint32 checksum) { checksum_ = checksum; }
 
   // Checksum methods to verify whether the ranges are corrupted (e.g. bad
   // memory access).
-  uint32 CalculateChecksum();
-  bool HasValidChecksum();
+  uint32 CalculateChecksum() const;
+  bool HasValidChecksum() const;
   void ResetChecksum();
 
   // Return true iff |other| object has same ranges_ as |this| object's ranges_.
   bool Equals(const BucketRanges* other) const;
 
  private:
   // A monotonically increasing list of values which determine which bucket to
   // put a sample into.  For each index, show the smallest sample that can be
   // added to the corresponding bucket.
   Ranges ranges_;
 
   // Checksum for the conntents of ranges_.  Used to detect random over-writes
   // of our data, and to quickly see if some other BucketRanges instance is
   // possibly Equal() to this instance.
   // TODO(kaiwang): Consider change this to uint64. Because we see a lot of
   // noise on UMA dashboard.
   uint32 checksum_;
 
   DISALLOW_COPY_AND_ASSIGN(BucketRanges);
 };
 
+//////////////////////////////////////////////////////////////////////////////
+// Expose only for test.
+BASE_EXPORT_PRIVATE extern const uint32 kCrcTable[256];
+
 }  // namespace base
 
 #endif  // BASE_METRICS_BUCKET_RANGES_H_