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..7c6b906cf59da131165315f4541fda24b3555f50 100644
--- a/base/metrics/histogram_snapshot_manager.cc
+++ b/base/metrics/histogram_snapshot_manager.cc
@@ -31,10 +31,11 @@ void HistogramSnapshotManager::StartDeltas() {
 
   DCHECK(owned_histograms_.empty());
 
-#ifdef DEBUG
-    CHECK(!iter->second.histogram);
-    CHECK(!iter->second.accumulated_samples);
-    CHECK(!(iter->second.inconsistencies &
+#if DCHECK_IS_ON()
+  for (const auto& hash_and_info : known_histograms_) {
+    CHECK(!hash_and_info.second.histogram);
+    CHECK(!hash_and_info.second.accumulated_samples);
+    CHECK(!(hash_and_info.second.inconsistencies &

[Ilya Sherman, 2016/05/05 07:22:37]

Should these be DCHECKs?  It's odd for them to be checks if they're only
executed when DCHECK is on.

[bcwhite, 2016/05/05 16:01:17]

They could be.  Didn't seem to be any difference.  Done.

             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_);