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..85139bd8b05f9e45aa6c95f2691b512f7be3e790
--- /dev/null
+++ b/base/test/histogram_recorder.h
@@ -0,0 +1,35 @@
+// 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(

[Nico, 2013/07/15 17:38:27]

nit: I'd give a function that allocates the return value a different name than
GetFoo (NewFoo?). But since it returns a scoped_ptr, feeel free to igore this.

[lpromero, 2013/07/17 13:40:48]

I see what you mean, especially with Cocoa guidelines :)
I did a quick check in the chromium tree:
$ git grep "scoped_ptr<" | grep "> Get" | wc -l  ## return 166
$ git grep "scoped_ptr<" | grep "> New" | wc -l  ## return 6

I'll go with the flow, since as you noted, the return type is scoped_ptr and
makes it clear what happens.

+      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_