Re-land https://codereview.chromium.org/23602005: Use nobarrier atomics for lossy counters in base:…

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 <string>
 
+#include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
 
 class Pickle;
 class PickleIterator;
 
 namespace base {
 
@@ skipping 23 matching lines @@
     PickleIterator* iter);
 
 // Create or find existing histogram and add the samples from pickle.
 // Silently returns when seeing any data problem in the pickle.
 BASE_EXPORT void DeserializeHistogramAndAddSamples(PickleIterator* iter);
 
 ////////////////////////////////////////////////////////////////////////////////
 
 class BASE_EXPORT HistogramBase {
  public:
-  typedef int Sample;  // Used for samples.
-  typedef int Count;   // Used to count samples.
+  typedef int Sample;                   // Used for samples.
+  typedef subtle::Atomic32 Count;     // Used to count samples.
 
   static const Sample kSampleType_MAX;  // INT_MAX
 
   enum Flags {
     kNoFlags = 0,
     kUmaTargetedHistogramFlag = 0x1,  // Histogram should be UMA uploaded.
 
     // Indicate that the histogram was pickled to be sent across an IPC Channel.
     // If we observe this flag on a histogram being aggregated into after IPC,
     // then we are running in a single process mode, and the aggregation should
@@ skipping 97 matching lines @@
  private:
   const std::string histogram_name_;
   int32 flags_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramBase);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_BASE_H_