Use nobarrier atomics for lossy counters in base::HistogramSamples and base::SampleVector

base/metrics/sample_vector.cc

Index: base/metrics/sample_vector.cc
===================================================================
--- base/metrics/sample_vector.cc	(revision 219973)
+++ base/metrics/sample_vector.cc	(working copy)
@@ -24,7 +24,8 @@
 
 void SampleVector::Accumulate(Sample value, Count count) {
   size_t bucket_index = GetBucketIndex(value);
-  counts_[bucket_index] += count;
+  subtle::NoBarrier_Store(&counts_[bucket_index],
+      subtle::NoBarrier_Load(&counts_[bucket_index]) + count);
   IncreaseSum(count * value);
   IncreaseRedundantCount(count);
 }