Use subtle::NoBarrier* statements for Histogram flags_ field.

base/metrics/histogram_base.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_BASE_H_
 #define BASE_METRICS_HISTOGRAM_BASE_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 87 matching lines @@
   virtual ~HistogramBase();
 
   const std::string& histogram_name() const { return histogram_name_; }
 
   // Comapres |name| to the histogram name and triggers a DCHECK if they do not
   // match. This is a helper function used by histogram macros, which results in
   // in more compact machine code being generated by the macros.
   void CheckName(const StringPiece& name) const;
 
   // Operations with Flags enum.
-  int32_t flags() const { return flags_; }
+  int32_t flags() const { return subtle::NoBarrier_Load(&flags_); }
   void SetFlags(int32_t flags);
   void ClearFlags(int32_t flags);
 
   virtual HistogramType GetHistogramType() const = 0;
 
   // Whether the histogram has construction arguments as parameters specified.
   // For histograms that don't have the concept of minimum, maximum or
   // bucket_count, this function always returns false.
   virtual bool HasConstructionArguments(Sample expected_minimum,
                                         Sample expected_maximum,
@@ skipping 63 matching lines @@
   void WriteAsciiBucketValue(Count current,
                              double scaled_sum,
                              std::string* output) const;
 
   // Retrieves the callback for this histogram, if one exists, and runs it
   // passing |sample| as the parameter.
   void FindAndRunCallback(Sample sample) const;
 
  private:
   const std::string histogram_name_;
-  int32_t flags_;
+  AtomicCount flags_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramBase);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_BASE_H_