Add a HistogramRecorder class and use cases.

base/test/histogram_recorder.h

Index: base/test/histogram_recorder.h
diff --git a/base/test/histogram_recorder.h b/base/test/histogram_recorder.h
new file mode 100644
index 0000000000000000000000000000000000000000..9edd982134f0783dd0f414dd954eb1b458733531
--- /dev/null
+++ b/base/test/histogram_recorder.h
@@ -0,0 +1,37 @@
+// 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/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:
+  // Constructor takes a histogram prefix.

[blundell, 2013/07/10 14:35:36]

Instead of this comment, have a comment explaining what |prefix| is used for.

[lpromero, 2013/07/10 17:08:33]

Done.

+  explicit HistogramRecorder(const std::string& prefix);
+  virtual ~HistogramRecorder();
+
+  // Returns the histogram data accumulated since this instance was created.
+  // The returned pointer is owned by the HistogramRecorder instance and cannot
+  // be used after that instance is destroyed.
+  // Returns NULL if no samples are available.
+  HistogramSamples* GetHistogramSamples(const std::string& histogram_name);

[blundell, 2013/07/10 14:35:36]

I think this would be more clear as GetHistogramSamplesSinceCreation()

[lpromero, 2013/07/10 17:08:33]

Done.

+
+ private:
+  // Used to determine the histogram changes made during this instance's
+  // lifecycle.
+  std::map<std::string, HistogramSamples*> original_samples_;
+  std::map<std::string, HistogramSamples*> samples_;
+};
+
+}  // namespace base
+
+#endif  // BASE_TEST_HISTOGRAM_RECORDER_H_