Support saving browser metrics to disk and reading them during next run.

base/metrics/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.
 
 // Histogram is an object that aggregates statistics, and can summarize them in
 // various forms, including ASCII graphical, HTML, and numerically (as a
 // vector of numbers corresponding to each of the aggregating buckets).
 
 // It supports calls to accumulate either time intervals (which are processed
 // as integral number of milliseconds), or arbitrary integral units.
@@ skipping 186 matching lines @@
   // HistogramBase implementation:
   uint64_t name_hash() const override;
   HistogramType GetHistogramType() const override;
   bool HasConstructionArguments(Sample expected_minimum,
                                 Sample expected_maximum,
                                 uint32_t expected_bucket_count) const override;
   void Add(Sample value) override;
   void AddCount(Sample value, int count) override;
   std::unique_ptr<HistogramSamples> SnapshotSamples() const override;
   std::unique_ptr<HistogramSamples> SnapshotDelta() override;
+  std::unique_ptr<HistogramSamples> SnapshotFinalDelta() const override;
   void AddSamples(const HistogramSamples& samples) override;
   bool AddSamplesFromPickle(base::PickleIterator* iter) override;
   void WriteHTMLGraph(std::string* output) const override;
   void WriteAscii(std::string* output) const override;
 
  protected:
   // This class, defined entirely within the .cc file, contains all the
   // common logic for building a Histogram and can be overridden by more
   // specific types to alter details of how the creation is done. It is
   // defined as an embedded class (rather than an anonymous one) so it
@@ skipping 89 matching lines @@
   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<SampleVector> samples_;
 
   // Also keep a previous uploaded state for calculating deltas.
   std::unique_ptr<HistogramSamples> logged_samples_;
 
+  // Flag to indicate if PrepareFinalDelta has been previously called. It is
+  // used to DCHECK that a final delta is not created multiple times.
+  mutable bool final_delta_created_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(Histogram);
 };
 
 //------------------------------------------------------------------------------
 
 // LinearHistogram is a more traditional histogram, with evenly spaced
 // buckets.
 class BASE_EXPORT LinearHistogram : public Histogram {
  public:
   ~LinearHistogram() override;
@@ skipping 212 matching lines @@
   static HistogramBase* DeserializeInfoImpl(base::PickleIterator* iter);
 
   static bool ValidateCustomRanges(const std::vector<Sample>& custom_ranges);
 
   DISALLOW_COPY_AND_ASSIGN(CustomHistogram);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_H_