Change some operations touching counts_[] to be atomic.

base/metrics/sample_vector.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.
 
 // SampleVector implements HistogramSamples interface. It is used by all
 // Histogram based classes to store samples.
 
 #ifndef BASE_METRICS_SAMPLE_VECTOR_H_
 #define BASE_METRICS_SAMPLE_VECTOR_H_
 
@@ skipping 28 matching lines @@
  protected:
   virtual bool AddSubtractImpl(
       SampleCountIterator* iter,
       HistogramSamples::Operator op) OVERRIDE;  // |op| is ADD or SUBTRACT.
 
   virtual size_t GetBucketIndex(HistogramBase::Sample value) const;
 
  private:
   FRIEND_TEST_ALL_PREFIXES(HistogramTest, CorruptSampleCounts);
 
-  std::vector<HistogramBase::Count> counts_;
+  std::vector<HistogramBase::AtomicCount> counts_;
 
   // Shares the same BucketRanges with Histogram object.
   const BucketRanges* const bucket_ranges_;
 
   DISALLOW_COPY_AND_ASSIGN(SampleVector);
 };
 
 class BASE_EXPORT_PRIVATE SampleVectorIterator : public SampleCountIterator {
  public:
-  SampleVectorIterator(const std::vector<HistogramBase::Count>* counts,
+  SampleVectorIterator(const std::vector<HistogramBase::AtomicCount>* counts,
                        const BucketRanges* bucket_ranges);
   virtual ~SampleVectorIterator();
 
   // SampleCountIterator implementation:
   virtual bool Done() const OVERRIDE;
   virtual void Next() OVERRIDE;
   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:
   void SkipEmptyBuckets();
 
-  const std::vector<HistogramBase::Count>* counts_;
+  const std::vector<HistogramBase::AtomicCount>* counts_;
   const BucketRanges* bucket_ranges_;
 
   size_t index_;
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_SAMPLE_VECTOR_H_