[histogram] Make histograms more resistant to overflows.

base/metrics/histogram.h

Index: base/metrics/histogram.h
diff --git a/base/metrics/histogram.h b/base/metrics/histogram.h
index 503d246379c01099fe91b1447587c56ccfb0b5bf..5baf8e8b6f98d14835816b63e33fd69de474dc15 100644
--- a/base/metrics/histogram.h
+++ b/base/metrics/histogram.h
@@ -267,6 +267,10 @@ class BASE_EXPORT Histogram : public HistogramBase {
   // Implementation of SnapshotSamples function.
   std::unique_ptr<SampleVector> SnapshotSampleVector() const;
 
+  // Create a copy of given samples.
+  std::unique_ptr<SampleVector> CloneSampleVector(
+      const HistogramSamples& samples) const;
+
   //----------------------------------------------------------------------------
   // Helpers for emitting Ascii graphic.  Each method appends data to output.
 
@@ -303,11 +307,10 @@ class BASE_EXPORT Histogram : public HistogramBase {
   Sample declared_min_;  // Less than this goes into the first bucket.
   Sample declared_max_;  // Over this goes into the last bucket.
 
-  // Finally, provide the state that changes with the addition of each new
-  // sample.
-  std::unique_ptr<SampleVectorBase> samples_;
+  // Samples that have not yet been logged with SnapshotDelta().
+  std::unique_ptr<HistogramSamples> unlogged_samples_;
 
-  // Also keep a previous uploaded state for calculating deltas.
+  // Accumulation of all samples that have logged with SnapshotDelta().
   std::unique_ptr<HistogramSamples> logged_samples_;
 
   // Flag to indicate if PrepareFinalDelta has been previously called. It is