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"
+
+// ChromeHistogramTester extends the base class by gathering in histogram data
+// from all other processes before performing assertions.

[Ilya Sherman, 2014/07/16 20:01:13]

IMO, it would be better to move this comment to be part of the documentation for
FetchTestingSnapshot, since the other methods (from the base class) are not
affected.  If you prefer to also have a class-level comment that's fine; in that
case, please provide documentation in both places.

[Mike Lerman, 2014/07/17 15:04:18]

Done.

+class ChromeHistogramTester : public base::HistogramTester {
+ public:
+  // ChromeHistogramTester should be created before the execution of the test
+  // case.
+  ChromeHistogramTester();
+

[Ilya Sherman, 2014/07/16 20:01:13]

Optional nit: I'd omit this blank line.

[Mike Lerman, 2014/07/17 15:04:18]

Ya, the standard seems to be constructor-destructor without no newline between.

+  ~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.
+  void FetchTestingSnapshot();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ChromeHistogramTester);
+};
+
+#endif  // CHROME_TEST_BASE_CHROME_HISTOGRAM_TESTER_H_