Add a HistogramRecorder class and use cases.

base/test/histogram_recorder.cc

+// 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.
+
+#include "base/test/histogram_recorder.h"
+
+#include "base/metrics/statistics_recorder.h"
+#include "base/stl_util.h"
+
+namespace base {
+
+HistogramRecorder::HistogramRecorder(
+    const std::string& prefix) {
+  // Record any histogram data that exists when the object is created so it can
+  // be subtracted later.
+  StatisticsRecorder::Initialize();
+  StatisticsRecorder::Histograms histograms;
+  StatisticsRecorder::GetSnapshot(prefix, &histograms);
+  for (size_t i = 0; i < histograms.size(); i++) {
+    original_samples_[histograms[i]->histogram_name()] =
+        histograms[i]->SnapshotSamples().release();
+  }
+}
+
+HistogramRecorder::~HistogramRecorder() {
+  STLDeleteValues(&original_samples_);
+  STLDeleteValues(&samples_);
+}
+
+HistogramSamples* HistogramRecorder::GetHistogramSamples(
+    const std::string& histogram_name) {
+  HistogramBase* histogram = StatisticsRecorder::FindHistogram(histogram_name);
+  if (!histogram)
+    return NULL;
+  HistogramSamples* named_samples = histogram->SnapshotSamples().release();
+  HistogramSamples* named_original_samples = original_samples_[histogram_name];
+  if (named_original_samples)
+    named_samples->Subtract(*named_original_samples);
+  samples_[histogram_name] = named_samples;
+  return named_samples;
+}
+
+}  // namespace base