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..bf3c3ab6cfd317d9c366e25504b4e746c6382597 100644
--- a/base/metrics/histogram.cc
+++ b/base/metrics/histogram.cc
@@ -453,6 +453,15 @@ std::unique_ptr<HistogramSamples> Histogram::SnapshotDelta() {
   return snapshot;
 }
 
+std::unique_ptr<HistogramSamples> Histogram::SnapshotDifference() const {
+  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);
 }