Add a HistogramRecorder class and use cases.

base/test/histogram_recorder.cc

+// Copyright 2013 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/histogram_recorder.h"
+
+#include "base/metrics/histogram.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/stl_util.h"
+
+namespace base {
+
+HistogramRecorder::HistogramRecorder() {
+  // Record any histogram data that exists when the object is created so it can
+  // be subtracted later.
+  StatisticsRecorder::Initialize();
+  StatisticsRecorder::Histograms histograms;
+  StatisticsRecorder::GetSnapshot("", &histograms);
+  for (size_t i = 0; i < histograms.size(); i++) {
+    original_samples_[histograms[i]->histogram_name()] =
+        histograms[i]->SnapshotSamples().release();
+  }
+}
+
+HistogramRecorder::~HistogramRecorder() {
+  STLDeleteValues(&original_samples_);
+}
+
+scoped_ptr<HistogramSamples>
+    HistogramRecorder::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);

[blundell, 2013/07/12 13:22:42]

I believe that as of pretty recently, you are supposed to (and will soon be
required to) use .Get() when using a scoped_ptr as the underlying pointer. I
didn't check the rest of this CL for any other such usages.

[lpromero, 2013/07/17 13:40:48]

Done.

[Ryan Sleevi, 2013/07/17 17:03:26]

Can you please point where?

scoped_ptr<> is intended to be used as an equivalent (drop-in) replacement for
C++11's unique_ptr<>.

unique_ptr<> has an operator ->(), as defined in 20.7.1.2.4, so you shouldn't
need to add any .get() types.

[lpromero, 2013/07/18 07:43:37]

Done.

[blundell, 2013/07/18 07:43:54]

Oops, I had misunderstood what was going on in crbug.com/110610. Thanks for the
correction!

On 2013/07/17 17:03:26, Ryan Sleevi wrote:

+  return named_samples.Pass();
+}
+
+}  // namespace base