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 e2bf4bfed18a949d8a95177287f9c432541c731a..a2dbb61b218b9b42b90543547c344fb42d26b5cc 100644
--- a/base/metrics/sparse_histogram.cc
+++ b/base/metrics/sparse_histogram.cc
@@ -126,6 +126,8 @@ std::unique_ptr<HistogramSamples> SparseHistogram::SnapshotSamples() const {
 }
 
 std::unique_ptr<HistogramSamples> SparseHistogram::SnapshotDelta() {
+  DCHECK(!final_delta_created_);
+
   std::unique_ptr<SampleMap> snapshot(new SampleMap(name_hash()));
   base::AutoLock auto_lock(lock_);
   snapshot->Add(*samples_);
@@ -136,6 +138,19 @@ std::unique_ptr<HistogramSamples> SparseHistogram::SnapshotDelta() {
   return std::move(snapshot);
 }
 
+std::unique_ptr<HistogramSamples> SparseHistogram::SnapshotFinalDelta() const {
+  DCHECK(!final_delta_created_);
+  final_delta_created_ = true;
+
+  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);