Change some operations touching counts_[] to be atomic.

base/metrics/histogram_samples.h

Index: base/metrics/histogram_samples.h
===================================================================
--- base/metrics/histogram_samples.h	(revision 242425)
+++ base/metrics/histogram_samples.h	(working copy)
@@ -39,7 +39,9 @@
 
   // 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.