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..dc6cb8a86ed62c97cce03ecb5cc88676ba1b4282 100644
--- a/base/metrics/histogram_snapshot_manager.cc
+++ b/base/metrics/histogram_snapshot_manager.cc
@@ -31,11 +31,12 @@ void HistogramSnapshotManager::StartDeltas() {
 
   DCHECK(owned_histograms_.empty());
 
-#ifdef DEBUG
-    CHECK(!iter->second.histogram);
-    CHECK(!iter->second.accumulated_samples);
-    CHECK(!(iter->second.inconsistencies &
-            HistogramBase::NEW_INCONSISTENCY_FOUND));
+#if DCHECK_IS_ON()
+  for (const auto& hash_and_info : known_histograms_) {
+    DCHECK(!hash_and_info.second.histogram);
+    DCHECK(!hash_and_info.second.accumulated_samples);
+    DCHECK(!(hash_and_info.second.inconsistencies &
+             HistogramBase::NEW_INCONSISTENCY_FOUND));
   }
 #endif
 }
@@ -60,6 +61,12 @@ void HistogramSnapshotManager::PrepareAbsoluteTakingOwnership(
   owned_histograms_.push_back(std::move(histogram));
 }
 
+void HistogramSnapshotManager::PrepareFinalDeltaTakingOwnership(
+    std::unique_ptr<const HistogramBase> histogram) {
+  PrepareSamples(histogram.get(), histogram->SnapshotFinalDelta());
+  owned_histograms_.push_back(std::move(histogram));
+}
+
 void HistogramSnapshotManager::FinishDeltas() {
   DCHECK(preparing_deltas_);