Add a StatisticsDeltaReader class and use cases

base/test/statistics_delta_reader.h

Index: base/test/statistics_delta_reader.h
diff --git a/base/test/statistics_delta_reader.h b/base/test/statistics_delta_reader.h
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+++ b/base/test/statistics_delta_reader.h
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+// Copyright 2014 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_TEST_STATISTICS_DELTA_READER_H_
+#define BASE_TEST_STATISTICS_DELTA_READER_H_
+
+#include <map>
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram_samples.h"
+
+namespace base {
+
+// This class acts as a differential reader for histogram samples, enabling
+// tests to check that metrics were recorded as they should be.
+// Before using this class, StatisticsRecoder must be initialized.
+class StatisticsDeltaReader {
+ public:
+  StatisticsDeltaReader();
+  ~StatisticsDeltaReader();
+
+  // Returns the histogram data accumulated since this instance was created.
+  // Returns NULL if no samples are available.
+  scoped_ptr<HistogramSamples> GetHistogramSamplesSinceCreation(
+      const std::string& histogram_name);
+
+ private:
+  // Used to determine the histogram changes made during this instance's
+  // lifecycle. This instance takes ownership of the samples, which are deleted
+  // when the instance is destroyed.
+  std::map<std::string, HistogramSamples*> original_samples_;
+
+  DISALLOW_COPY_AND_ASSIGN(StatisticsDeltaReader);
+};
+
+}  // namespace base
+
+#endif  // BASE_TEST_STATISTICS_DELTA_READER_H_