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

base/metrics/histogram.h

Index: base/metrics/histogram.h
diff --git a/base/metrics/histogram.h b/base/metrics/histogram.h
index d4d009c6ae01c1fa246c5e3a364969899a39aa4d..2562e3c51844e44a1f7f806cd00a5b9454be0c78 100644
--- a/base/metrics/histogram.h
+++ b/base/metrics/histogram.h
@@ -204,6 +204,7 @@ class BASE_EXPORT Histogram : public HistogramBase {
   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;
@@ -313,6 +314,9 @@ class BASE_EXPORT Histogram : public HistogramBase {
   // Also keep a previous uploaded state for calculating deltas.
   std::unique_ptr<HistogramSamples> logged_samples_;
 
+  // Flag to indicate if PrepareFinalDelta has been previously called.

[Alexei Svitkine (slow), 2016/05/09 21:24:10]

Nit: Maybe mention why we need this. (i.e. used for DCHECKs.)

[bcwhite, 2016/05/10 15:42:40]

Done.

+  mutable bool final_delta_created_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(Histogram);
 };