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

base/metrics/histogram.cc

Index: base/metrics/histogram.cc
diff --git a/base/metrics/histogram.cc b/base/metrics/histogram.cc
index 86c6746c0f590199cefe57c21d35ed4fcb7bd3a2..3571657513f6482c5724f721c17e8a2c354ab79a 100644
--- a/base/metrics/histogram.cc
+++ b/base/metrics/histogram.cc
@@ -439,6 +439,8 @@ std::unique_ptr<HistogramSamples> Histogram::SnapshotSamples() const {
 }
 
 std::unique_ptr<HistogramSamples> Histogram::SnapshotDelta() {
+  DCHECK(!final_delta_created_);
+
   std::unique_ptr<HistogramSamples> snapshot = SnapshotSampleVector();
   if (!logged_samples_) {
     // If nothing has been previously logged, save this one as
@@ -453,6 +455,18 @@ std::unique_ptr<HistogramSamples> Histogram::SnapshotDelta() {
   return snapshot;
 }
 
+std::unique_ptr<HistogramSamples> Histogram::SnapshotFinalDelta() const {
+  DCHECK(!final_delta_created_);
+  final_delta_created_ = true;
+
+  std::unique_ptr<HistogramSamples> snapshot = SnapshotSampleVector();
+
+  // Subtract what was previously logged and then return.
+  if (logged_samples_)
+    snapshot->Subtract(*logged_samples_);
+  return snapshot;
+}
+
 void Histogram::AddSamples(const HistogramSamples& samples) {
   samples_->Add(samples);
 }