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

base/metrics/histogram_snapshot_manager.h

 // 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.
 
 #ifndef BASE_METRICS_HISTOGRAM_SNAPSHOT_MANAGER_H_
 #define BASE_METRICS_HISTOGRAM_SNAPSHOT_MANAGER_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 44 matching lines @@
     FinishDeltas();
   }
 
   // When the collection is not so simple as can be done using a single
   // iterator, the steps can be performed separately. Call PerpareDelta()
   // as many times as necessary with a single StartDeltas() before and
   // a single FinishDeltas() after. All passed histograms must live
   // until FinishDeltas() completes. PrepareAbsolute() works the same
   // but assumes there were no previous logged values and no future deltas
   // will be created (and thus can work on read-only histograms).
+  // PrepareFinalDelta() works like PrepareDelta() except that it does
+  // not update the previous logged values and can thus be used with
+  // read-only files.
   // Use Prepare*TakingOwnership() if it is desireable to have this class
   // automatically delete the histogram once it is "finished".
   void StartDeltas();
   void PrepareDelta(HistogramBase* histogram);
   void PrepareDeltaTakingOwnership(std::unique_ptr<HistogramBase> histogram);
   void PrepareAbsolute(const HistogramBase* histogram);
   void PrepareAbsoluteTakingOwnership(
       std::unique_ptr<const HistogramBase> histogram);
+  void PrepareFinalDelta(const HistogramBase* histogram);

[Ilya Sherman, 2016/05/04 06:48:07]

nit: Only the TakingOwnership() method is used, so please omit this method.  It
can always be added later if a use case comes up for it.

[bcwhite, 2016/05/04 14:12:50]

<shudder>  I hate inconsistent APIs.  But done.

+  void PrepareFinalDeltaTakingOwnership(
+      std::unique_ptr<const HistogramBase> histogram);
   void FinishDeltas();
 
  private:
   FRIEND_TEST_ALL_PREFIXES(HistogramSnapshotManagerTest, CheckMerge);
 
   // During a snapshot, samples are acquired and aggregated. This structure
   // contains all the information collected for a given histogram. Once a
   // snapshot operation is finished, it is generally emptied except for
   // information that must persist from one report to the next, such as
   // the "inconsistencies".
@@ skipping 38 matching lines @@
   // |histogram_flattener_| handles the logistics of recording the histogram
   // deltas.
   HistogramFlattener* histogram_flattener_;  // Weak.
 
   DISALLOW_COPY_AND_ASSIGN(HistogramSnapshotManager);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_SNAPSHOT_MANAGER_H_