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..6f58e8c11c734b5ac03bf4d65f3805bc263711b3 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> SnapshotDifference() const override;
   void AddSamples(const HistogramSamples& samples) override;
   bool AddSamplesFromPickle(base::PickleIterator* iter) override;
   void WriteHTMLGraph(std::string* output) const override;