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

base/metrics/histogram_base.h

Index: base/metrics/histogram_base.h
diff --git a/base/metrics/histogram_base.h b/base/metrics/histogram_base.h
index 669d4bac440bb45f1abf0e1093d09d67bd5ae86e..d246691ca257093851f7dc7c6393d7af51a3285b 100644
--- a/base/metrics/histogram_base.h
+++ b/base/metrics/histogram_base.h
@@ -105,7 +105,7 @@ class BASE_EXPORT HistogramBase {
   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);
 
@@ -189,7 +189,7 @@ class BASE_EXPORT HistogramBase {
 
  private:
   const std::string histogram_name_;
-  int32_t flags_;
+  AtomicCount flags_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramBase);
 };