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

base/metrics/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.
 
 // Histogram is an object that aggregates statistics, and can summarize them in
 // various forms, including ASCII graphical, HTML, and numerically (as a
 // vector of numbers corresponding to each of the aggregating buckets).
 // See header file for details and examples.
 
 #include "base/metrics/histogram.h"
@@ skipping 435 matching lines @@
     logged_samples_.swap(snapshot);
     return SnapshotSampleVector();
   }
 
   // Subtract what was previously logged and update that information.
   snapshot->Subtract(*logged_samples_);
   logged_samples_->Add(*snapshot);
   return snapshot;
 }
 
+std::unique_ptr<HistogramSamples> Histogram::SnapshotDifference() const {
+  std::unique_ptr<HistogramSamples> snapshot = SnapshotSampleVector();
+
+  // Subtract what was previously logged and then return.
+  if (logged_samples_)
+    snapshot->Subtract(*logged_samples_);
+  return snapshot;
+}
+
 void Histogram::AddSamples(const HistogramSamples& samples) {
   samples_->Add(samples);
 }
 
 bool Histogram::AddSamplesFromPickle(PickleIterator* iter) {
   return samples_->AddFromPickle(iter);
 }
 
 // The following methods provide a graphical histogram display.
 void Histogram::WriteHTMLGraph(std::string* output) const {
@@ skipping 699 matching lines @@
     Sample sample = custom_ranges[i];
     if (sample < 0 || sample > HistogramBase::kSampleType_MAX - 1)
       return false;
     if (sample != 0)
       has_valid_range = true;
   }
   return has_valid_range;
 }
 
 }  // namespace base