Rework UMAHistogramHelper and StatisticsDeltaReader into [Chrome]HistogramTester.

chrome/test/base/chrome_histogram_tester.cc

+// 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.
+
+#include "chrome/test/base/chrome_histogram_tester.h"
+
+#include "base/bind.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/test/test_timeouts.h"
+#include "chrome/test/base/ui_test_utils.h"
+#include "content/public/browser/histogram_fetcher.h"
+#include "testing/gtest/include/gtest/gtest.h"

[Ilya Sherman, 2014/07/15 03:56:35]

nit: Is this include needed?

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

Nope!

+
+ChromeHistogramTester::ChromeHistogramTester() {
+  base::StatisticsRecorder::Initialize();

[Ilya Sherman, 2014/07/15 03:56:35]

This is already called by the base class's constructor, right?

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

Done.

+}
+
+ChromeHistogramTester::~ChromeHistogramTester() {
+}
+
+
+void ChromeHistogramTester::FetchTestingSnapshot() {
+  base::Closure callback = base::Bind(&ChromeHistogramTester::FetchCallback,
+                                      base::Unretained(this));
+
+  content::FetchHistogramsAsynchronously(
+      base::MessageLoop::current(),
+      callback,
+      // If this call times out, it means that a child process is not
+      // responding, which is something we should not ignore.  The timeout is
+      // set to be longer than the normal browser test timeout so that it will
+      // be prempted by the normal timeout.
+      TestTimeouts::action_max_timeout() * 2);
+  content::RunMessageLoop();

[Ilya Sherman, 2014/07/15 03:56:35]

Is it possible for content::FetchHistogramsAsynchronously() to synchronously
call back into the callback?  If so, this is a race condition: You might try to
quit the message loop prior to running it, and then run it with no Quit()
callback in sight.  Using a MessageLoopRunner would be better.

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

I don't think there's a way for it to be run synchronously. Even if
FetchTestingSnapshot() is called twice, the tasks are still posted to the
MessageLoop. But I don't know a lot about the MessageLoop - I'll look into the
MessageLoopRunner.

I think I made the change you requested - the NaCl tests still past. Please make
sure I understand how this was supposed to work correctly.

+}
+
+
+void ChromeHistogramTester::FetchCallback() {
+  base::MessageLoopForUI::current()->Quit();
+}