Change some operations touching counts_[] to be atomic.

base/metrics/histogram_samples.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.
 
 #ifndef BASE_METRICS_HISTOGRAM_SAMPLES_H_
 #define BASE_METRICS_HISTOGRAM_SAMPLES_H_
 
 #include "base/basictypes.h"
 #include "base/metrics/histogram_base.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 21 matching lines @@
   // Add from serialized samples.
   virtual bool AddFromPickle(PickleIterator* iter);
 
   virtual void Subtract(const HistogramSamples& other);
 
   virtual scoped_ptr<SampleCountIterator> Iterator() const = 0;
   virtual bool Serialize(Pickle* pickle) const;
 
   // Accessor fuctions.
   int64 sum() const { return sum_; }
-  HistogramBase::Count redundant_count() const { return redundant_count_; }
+  HistogramBase::Count redundant_count() const {
+    return subtle::NoBarrier_Load(&redundant_count_);
+  }
 
  protected:
   // 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);
 
  private:
@@ skipping 25 matching lines @@
 
   // Get the index of current histogram bucket.
   // For histograms that don't use predefined buckets, it returns false.
   // Requires: !Done();
   virtual bool GetBucketIndex(size_t* index) const;
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_SAMPLES_H_