Support "shared" histograms between processes.

base/metrics/histogram_delta_serialization.h

 // Copyright 2013 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_DELTA_SERIALIZATION_H_
 #define BASE_METRICS_HISTOGRAM_DELTA_SERIALIZATION_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 10 matching lines @@
 
 // Serializes and restores histograms deltas.
 class BASE_EXPORT HistogramDeltaSerialization : public HistogramFlattener {
  public:
   // |caller_name| is string used in histograms for counting inconsistencies.
   explicit HistogramDeltaSerialization(const std::string& caller_name);
   ~HistogramDeltaSerialization() override;
 
   // Computes deltas in histogram bucket counts relative to the previous call to
   // this method. Stores the deltas in serialized form into |serialized_deltas|.
   // If |serialized_deltas| is NULL, no data is serialized, though the next call

[Alexei Svitkine (slow), 2016/01/18 19:25:35]

Nit: "is NULL" -> "is null" per new conventions since you're updating the
comment.

[bcwhite, 2016/01/22 15:24:53]

Done.

-  // will compute the deltas relative to this one.
-  void PrepareAndSerializeDeltas(std::vector<std::string>* serialized_deltas);
+  // will compute the deltas relative to this one. Setting |include_persistent|
+  // will include histograms held in persistent memory (and thus may be reported
+  // elsewhere); otherwise only histograms local to this process are serialized.
+  void PrepareAndSerializeDeltas(std::vector<std::string>* serialized_deltas,
+                                 bool include_persistent);
 
   // Deserialize deltas and add samples to corresponding histograms, creating
   // them if necessary. Silently ignores errors in |serialized_deltas|.
   static void DeserializeAndAddSamples(
       const std::vector<std::string>& serialized_deltas);
 
  private:
   // HistogramFlattener implementation.
   void RecordDelta(const HistogramBase& histogram,
                    const HistogramSamples& snapshot) override;
@@ skipping 14 matching lines @@
   HistogramBase* inconsistencies_histogram_;
   HistogramBase* inconsistencies_unique_histogram_;
   HistogramBase* inconsistent_snapshot_histogram_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramDeltaSerialization);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_DELTA_SERIALIZATION_H_