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

base/metrics/sparse_histogram.cc

Index: base/metrics/sparse_histogram.cc
diff --git a/base/metrics/sparse_histogram.cc b/base/metrics/sparse_histogram.cc
index 31fcfd93d63cc4d1ee14667433488510785da7ad..f1c1d6963e6ba7f12588b759286e952ffa2bbc6f 100644
--- a/base/metrics/sparse_histogram.cc
+++ b/base/metrics/sparse_histogram.cc
@@ -136,6 +136,16 @@ std::unique_ptr<HistogramSamples> SparseHistogram::SnapshotDelta() {
   return std::move(snapshot);
 }
 
+std::unique_ptr<HistogramSamples> SparseHistogram::SnapshotDifference() const {
+  std::unique_ptr<SampleMap> snapshot(new SampleMap(name_hash()));
+  base::AutoLock auto_lock(lock_);
+  snapshot->Add(*samples_);
+
+  // Subtract what was previously logged and then return.
+  snapshot->Subtract(*logged_samples_);
+  return std::move(snapshot);
+}
+
 void SparseHistogram::AddSamples(const HistogramSamples& samples) {
   base::AutoLock auto_lock(lock_);
   samples_->Add(samples);