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

base/metrics/sparse_histogram.h

Index: base/metrics/sparse_histogram.h
diff --git a/base/metrics/sparse_histogram.h b/base/metrics/sparse_histogram.h
index c0723715a46581c27381e9f434ea99766b8c4d4c..3b302d6f223f626cfa8f152048a08af8e1b9f1d5 100644
--- a/base/metrics/sparse_histogram.h
+++ b/base/metrics/sparse_histogram.h
@@ -81,6 +81,7 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
   bool AddSamplesFromPickle(base::PickleIterator* iter) override;
   std::unique_ptr<HistogramSamples> SnapshotSamples() const override;
   std::unique_ptr<HistogramSamples> SnapshotDelta() override;
+  std::unique_ptr<HistogramSamples> SnapshotFinalDelta() const override;
   void WriteHTMLGraph(std::string* output) const override;
   void WriteAscii(std::string* output) const override;
 
@@ -121,6 +122,9 @@ class BASE_EXPORT SparseHistogram : public HistogramBase {
   // 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> logged_samples_;