Support "shared" histograms between processes.

base/metrics/histogram_base.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_base.h"
 
 #include <limits.h>
 
 #include <utility>
 
 #include "base/json/json_string_value_serializer.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram.h"
+#include "base/metrics/histogram_persistence.h"
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/sparse_histogram.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/pickle.h"
 #include "base/process/process_handle.h"
 #include "base/strings/stringprintf.h"
+#include "base/synchronization/lock.h"

[Alexei Svitkine (slow), 2016/01/12 21:54:44]

Doesn't seem like this header or the one on line 15 is used anywhere in this
file?

[bcwhite, 2016/01/13 13:38:29]

Done.

 #include "base/values.h"
 
 namespace base {
 
 std::string HistogramTypeToString(HistogramType type) {
   switch (type) {
     case HISTOGRAM:
       return "HISTOGRAM";
     case LINEAR_HISTOGRAM:
       return "LINEAR_HISTOGRAM";
@@ skipping 126 matching lines @@
   return result;
 }
 
 void HistogramBase::WriteAsciiBucketValue(Count current,
                                           double scaled_sum,
                                           std::string* output) const {
   StringAppendF(output, " (%d = %3.1f%%)", current, current/scaled_sum);
 }
 
 }  // namespace base