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

base/metrics/sparse_histogram.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/sparse_histogram.h"
 
 #include <utility>
 
 #include "base/memory/ptr_util.h"
 #include "base/metrics/metrics_hashes.h"
@@ skipping 118 matching lines @@
   std::unique_ptr<SampleMap> snapshot(new SampleMap(name_hash()));
   base::AutoLock auto_lock(lock_);
   snapshot->Add(*samples_);
 
   // Subtract what was previously logged and update that information.
   snapshot->Subtract(*logged_samples_);
   logged_samples_->Add(*snapshot);
   return std::move(snapshot);
 }
 
+std::unique_ptr<HistogramSamples> SparseHistogram::SnapshotDifference() const {
+  std::unique_ptr<SampleMap> snapshot(new SampleMap(name_hash()));
+  base::AutoLock auto_lock(lock_);
+  snapshot->Add(*samples_);
+
+  // Subtract what was previously logged and then return.
+  snapshot->Subtract(*logged_samples_);
+  return std::move(snapshot);
+}
+
 void SparseHistogram::AddSamples(const HistogramSamples& samples) {
   base::AutoLock auto_lock(lock_);
   samples_->Add(samples);
 }
 
 bool SparseHistogram::AddSamplesFromPickle(PickleIterator* iter) {
   base::AutoLock auto_lock(lock_);
   return samples_->AddFromPickle(iter);
 }
 
@@ skipping 117 matching lines @@
                                        std::string* output) const {
   StringAppendF(output,
                 "Histogram: %s recorded %d samples",
                 histogram_name().c_str(),
                 total_count);
   if (flags() & ~kHexRangePrintingFlag)
     StringAppendF(output, " (flags = 0x%x)", flags() & ~kHexRangePrintingFlag);
 }
 
 }  // namespace base