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

base/metrics/histogram_base.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_BASE_H_
 #define BASE_METRICS_HISTOGRAM_BASE_H_
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
@@ skipping 184 matching lines @@
 
   // Snapshot the current complete set of sample data.
   // Override with atomic/locked snapshot if needed.
   virtual std::unique_ptr<HistogramSamples> SnapshotSamples() const = 0;
 
   // Calculate the change (delta) in histogram counts since the previous call
   // to this method. Each successive call will return only those counts
   // changed since the last call.
   virtual std::unique_ptr<HistogramSamples> SnapshotDelta() = 0;
 
+  // Calculate the change (delta) in histogram counts since the previous call
+  // to SnapshotDelta() but do so without modifying any internal data as to
+  // what was previous logged. After such a call, no further calls to this
+  // method or to SnapshotDelta() should be done as the result would include
+  // data previously returned. Because no internal data is changed, this call
+  // can be made on "const" histograms such as those with data held in
+  // read-only memory.
+  virtual std::unique_ptr<HistogramSamples> SnapshotFinalDelta() const = 0;
+
   // The following methods provide graphical histogram displays.
   virtual void WriteHTMLGraph(std::string* output) const = 0;
   virtual void WriteAscii(std::string* output) const = 0;
 
   // Produce a JSON representation of the histogram. This is implemented with
   // the help of GetParameters and GetCountAndBucketData; overwrite them to
   // customize the output.
   void WriteJSON(std::string* output) const;
 
   // This enables a histogram that reports the what types of histograms are
@@ skipping 50 matching lines @@
 
   const std::string histogram_name_;
   AtomicCount flags_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramBase);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_BASE_H_