Rework UMAHistogramHelper and StatisticsDeltaReader into [Chrome]HistogramTester.

base/test/statistics_delta_reader.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/metrics/histogram.h"
-#include "base/metrics/histogram_samples.h"
-#include "base/metrics/statistics_recorder.h"
-#include "base/stl_util.h"
-
-namespace base {
-
-StatisticsDeltaReader::StatisticsDeltaReader() {
-  StatisticsRecorder::Initialize();  // Safe to call multiple times.
-
-  // Record any histogram data that exists when the object is created so it can
-  // be subtracted later.
-  StatisticsRecorder::Histograms histograms;
-  StatisticsRecorder::GetSnapshot(std::string(), &histograms);
-  for (size_t i = 0; i < histograms.size(); ++i) {
-    original_samples_[histograms[i]->histogram_name()] =
-        histograms[i]->SnapshotSamples().release();
-  }
-}
-
-StatisticsDeltaReader::~StatisticsDeltaReader() {
-  STLDeleteValues(&original_samples_);
-}
-
-scoped_ptr<HistogramSamples>
-    StatisticsDeltaReader::GetHistogramSamplesSinceCreation(
-        const std::string& histogram_name) {
-  HistogramBase* histogram = StatisticsRecorder::FindHistogram(histogram_name);
-  if (!histogram)
-    return scoped_ptr<HistogramSamples>();
-  scoped_ptr<HistogramSamples> named_samples(histogram->SnapshotSamples());
-  HistogramSamples* named_original_samples = original_samples_[histogram_name];
-  if (named_original_samples)
-    named_samples->Subtract(*named_original_samples);
-  return named_samples.Pass();
-}
-
-}  // namespace base