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 allowing the MessageLoop
+// to be fully run by calling |FetchTestingSnapshot| before executing the
+// histogram tests.

[Ilya Sherman, 2014/07/16 18:44:47]

More precisely, the Chrome subclass calls out to other processes -- like any
renderer process -- and fetches histogram data from them.  The MessageLoop is
just a synchronization tool in this case.

[Mike Lerman, 2014/07/16 19:36:36]

I had thought half the benefit, since this is used in a multi-process scenario,
is also that you're guaranteed that the other processes have done their work,
and we are in fact ready to do assert the test values.

But comment changed.

[Ilya Sherman, 2014/07/16 20:01:12]

Yes, this is the entire benefit.  However, the other processes don't run on the
test's main message loop (well, unless there's some really egregious
single_process shenanigans going on).  We spin the MessageLoop essentially as a
way to wait: The MessageLoop itself isn't doing any work; it's just blocking the
rest of the test from running.  Once the Quit() closure is posted, the
MessageLoop stops the test from blocking, and allows the rest of the test code
to run.  That Quit() closure is only posted when we get a message back
indicating that the other processes are done doing their thing, so it has the
effect of serving as a synchronization tool: The test code that needs to wait on
other processes doesn't get to run until the other guys are ready.

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