Rework UMAHistogramHelper and StatisticsDeltaReader into [Chrome]HistogramTester.

base/test/statistics_delta_reader_unittest.cc

-// Copyright 2014 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 "base/test/statistics_delta_reader.h"
-
-#include "base/memory/scoped_ptr.h"
-#include "base/metrics/histogram.h"
-#include "base/metrics/histogram_samples.h"
-#include "testing/gtest/include/gtest/gtest.h"
-
-namespace base {
-
-TEST(StatisticsDeltaReaderTest, Scope) {
-  // Record a histogram before the creation of the recorder.
-  UMA_HISTOGRAM_BOOLEAN("Test", true);
-
-  StatisticsDeltaReader reader;
-
-  // Verify that no histogram is recorded.
-  scoped_ptr<HistogramSamples> samples(
-      reader.GetHistogramSamplesSinceCreation("Test"));
-  EXPECT_FALSE(samples);
-
-  // Record a histogram after the creation of the recorder.
-  UMA_HISTOGRAM_BOOLEAN("Test", true);
-
-  // Verify that one histogram is recorded.
-  samples = reader.GetHistogramSamplesSinceCreation("Test");
-  EXPECT_TRUE(samples);
-  EXPECT_EQ(1, samples->TotalCount());
-}
-
-}  // namespace base