[histogram] Make histograms more resistant to overflows.

base/metrics/sparse_histogram.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_SPARSE_HISTOGRAM_H_
 #define BASE_METRICS_SPARSE_HISTOGRAM_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 79 matching lines @@
 
   // For constuctor calling.
   friend class SparseHistogramTest;
 
   // Protects access to |samples_|.
   mutable base::Lock lock_;
 
   // Flag to indicate if PrepareFinalDelta has been previously called.
   mutable bool final_delta_created_ = false;
 
-  std::unique_ptr<HistogramSamples> samples_;
+  std::unique_ptr<HistogramSamples> unlogged_samples_;
   std::unique_ptr<HistogramSamples> logged_samples_;
 
   DISALLOW_COPY_AND_ASSIGN(SparseHistogram);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_SPARSE_HISTOGRAM_H_