Rework UMAHistogramHelper and StatisticsDeltaReader into [Chrome]HistogramTester.

chrome/test/base/chrome_histogram_tester.h

+// Copyright (c) 2012 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 CHROME_TEST_BASE_CHROME_HISTOGRAM_TESTER_H_
+#define CHROME_TEST_BASE_CHROME_HISTOGRAM_TESTER_H_
+
+#include "base/test/histogram_tester.h"
+
+#include "base/memory/linked_ptr.h"
+#include "base/metrics/histogram.h"
+#include "base/metrics/histogram_base.h"

[Ilya Sherman, 2014/07/15 03:56:36]

nit: Are these two includes needed?

[Mike Lerman, 2014/07/16 17:29:04]

These four aren't needed!

+#include "base/metrics/histogram_samples.h"
+
+// ChromeHistogramTester provides a simple interface for examining UMA
+// histograms. Tests can use this interface to verify that UMA data is getting
+// logged as intended.

[Ilya Sherman, 2014/07/15 03:56:36]

Please document how this class differs from base::HistogramTester.

[Mike Lerman, 2014/07/16 17:29:04]

Done.

+class ChromeHistogramTester : public base::HistogramTester {
+ public:
+  // ChromeHistogramTester should be created before the execution of the test
+  // case.
+  ChromeHistogramTester();
+
+  ~ChromeHistogramTester();
+
+  // Fetch the values to be tested. This must be called after the test code has
+  // been executed but before performing the necessary assertions. This may be
+  // called multiple times.
+  virtual void FetchTestingSnapshot() OVERRIDE;
+
+ private:
+  void FetchCallback();

[Ilya Sherman, 2014/07/15 03:56:36]

nit: Please document this method.  Also, perhaps "OnFetchCompleted()" would be a
better name for this method?

[Mike Lerman, 2014/07/16 17:29:04]

Done.

+
+  DISALLOW_COPY_AND_ASSIGN(ChromeHistogramTester);

[Ilya Sherman, 2014/07/15 03:56:36]

nit: This should be the last line in the class.

[Mike Lerman, 2014/07/16 17:29:04]

Done.

+
+  // The map from histogram names to their snapshots
+  std::map<std::string, linked_ptr<base::HistogramSamples> >
+      histogram_snapshots;

[Ilya Sherman, 2014/07/15 03:56:36]

Is this actually needed?  I don't see it being used anywhere.

[Mike Lerman, 2014/07/16 17:29:04]

Done.

+};
+
+#endif  // CHROME_TEST_BASE_CHROME_HISTOGRAM_TESTER_H_