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

base/metrics/histogram_snapshot_manager.h

Index: base/metrics/histogram_snapshot_manager.h
diff --git a/base/metrics/histogram_snapshot_manager.h b/base/metrics/histogram_snapshot_manager.h
index 42dbac225d830838db4f998020265ec8fe03e380..ca7e7afb9270f762bd4775f242d1df3698e8af15 100644
--- a/base/metrics/histogram_snapshot_manager.h
+++ b/base/metrics/histogram_snapshot_manager.h
@@ -62,6 +62,9 @@ class BASE_EXPORT HistogramSnapshotManager {
   // 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();
@@ -70,6 +73,9 @@ class BASE_EXPORT HistogramSnapshotManager {
   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: