Add a HistogramRecorder class and use cases.

base/test/histogram_recorder.h

+// 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.
+
+#ifndef BASE_TEST_HISTOGRAM_RECORDER_H_
+#define BASE_TEST_HISTOGRAM_RECORDER_H_
+
+#include <map>
+
+#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 sent as they should be.
+class HistogramRecorder {
+ public:
+  HistogramRecorder();
+  virtual ~HistogramRecorder();
+
+  // 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.
+  std::map<std::string, HistogramSamples*> original_samples_;
+};
+
+}  // namespace base
+
+#endif  // BASE_TEST_HISTOGRAM_RECORDER_H_