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

base/metrics/histogram_snapshot_manager.cc

Index: base/metrics/histogram_snapshot_manager.cc
diff --git a/base/metrics/histogram_snapshot_manager.cc b/base/metrics/histogram_snapshot_manager.cc
index e21d512daba271dc43248a9e63b598ce98d55472..c0f63d710e6108e04426af4cc16df7e60b0c89c8 100644
--- a/base/metrics/histogram_snapshot_manager.cc
+++ b/base/metrics/histogram_snapshot_manager.cc
@@ -60,6 +60,17 @@ void HistogramSnapshotManager::PrepareAbsoluteTakingOwnership(
   owned_histograms_.push_back(std::move(histogram));
 }
 
+void HistogramSnapshotManager::PrepareDifference(
+    const HistogramBase* histogram) {
+  PrepareSamples(histogram, histogram->SnapshotDifference());
+}
+
+void HistogramSnapshotManager::PrepareDifferenceTakingOwnership(
+    std::unique_ptr<const HistogramBase> histogram) {
+  PrepareSamples(histogram.get(), histogram->SnapshotDifference());
+  owned_histograms_.push_back(std::move(histogram));
+}
+
 void HistogramSnapshotManager::FinishDeltas() {
   DCHECK(preparing_deltas_);