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

base/metrics/histogram_snapshot_manager.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/metrics/histogram_snapshot_manager.h"
 
 #include <memory>
 
 #include "base/debug/alias.h"
 #include "base/metrics/histogram_flattener.h"
@@ skipping 42 matching lines @@
 void HistogramSnapshotManager::PrepareAbsolute(const HistogramBase* histogram) {
   PrepareSamples(histogram, histogram->SnapshotSamples());
 }
 
 void HistogramSnapshotManager::PrepareAbsoluteTakingOwnership(
     std::unique_ptr<const HistogramBase> histogram) {
   PrepareSamples(histogram.get(), histogram->SnapshotSamples());
   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_);
 
   // Iterate over all known histograms to see what should be recorded.
   for (auto& hash_and_info : known_histograms_) {
     SampleInfo* sample_info = &hash_and_info.second;
 
     // First, record any histograms in which corruption was detected.
     if (sample_info->inconsistencies & HistogramBase::NEW_INCONSISTENCY_FOUND) {
       sample_info->inconsistencies &= ~HistogramBase::NEW_INCONSISTENCY_FOUND;
@@ skipping 91 matching lines @@
 
   histogram_flattener_->InconsistencyDetectedInLoggedCount(discrepancy);
   if (discrepancy > Histogram::kCommonRaceBasedCountMismatch) {
     // Fix logged_samples.
     logged_samples->Subtract(*logged_samples);
     logged_samples->Add(new_snapshot);
   }
 }
 
 }  // namespace base