[histogram] Make histograms more resistant to overflows.

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 <limits.h>
 #include <stddef.h>
 #include <stdint.h>
@@ skipping 169 matching lines @@
   // Note: This only serializes the construction arguments of the histogram, but
   // does not serialize the samples.
   bool SerializeInfo(base::Pickle* pickle) const;
 
   // Try to find out data corruption from histogram and the samples.
   // The returned value is a combination of Inconsistency enum.
   virtual uint32_t FindCorruption(const HistogramSamples& samples) const;
 
   // Snapshot the current complete set of sample data.
   // Override with atomic/locked snapshot if needed.
+  // NOTE: this data can overflow for long-running sessions. It should be
+  // handled with care and this method is recommended to be used only
+  // in about:histograms and test code.
   virtual std::unique_ptr<HistogramSamples> SnapshotSamples() const = 0;
 
   // Calculate the change (delta) in histogram counts since the previous call
   // to this method. Each successive call will return only those counts
   // changed since the last call.
   virtual std::unique_ptr<HistogramSamples> SnapshotDelta() = 0;
 
   // Calculate the change (delta) in histogram counts since the previous call
   // to SnapshotDelta() but do so without modifying any internal data as to
   // what was previous logged. After such a call, no further calls to this
@@ skipping 66 matching lines @@
 
   const std::string histogram_name_;
   AtomicCount flags_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramBase);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_BASE_H_