Add a StatisticsDeltaReader class and use cases

This CL adds a utility class to streamline the task of testing that
metrics have been updated as expected. Specifically, the
StatisticsDeltaReader class allows a client to obtain the differential
value of a given histogram in the interval since the given
StatisticsDeltaReader instance was created.

This reverts commit 6a7dfb3cd638829a04e002ea9bc5b18ed3c9c03f.

BUG=232414
TBR=rsleevy, groby

Review URL: https://codereview.chromium.org/120753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243303 0039d316-1c4b-4281-b951-d872f2087c98

8 files changed, 197 insertions, 167 deletions

diff --git a/base/base.gyp b/base/base.gyp
index 7c2b54d..96ed56f 100644
--- a/base/base.gyp
+++ b/base/base.gyp
@@ -608,6 +608,7 @@
         'template_util_unittest.cc',
         'test/expectations/expectation_unittest.cc',
         'test/expectations/parser_unittest.cc',
+        'test/statistics_delta_reader_unittest.cc',
         'test/test_reg_util_win_unittest.cc',
         'test/trace_event_analyzer_unittest.cc',
         'threading/non_thread_safe_unittest.cc',
@@ -914,6 +915,8 @@
         'test/simple_test_clock.h',
         'test/simple_test_tick_clock.cc',
         'test/simple_test_tick_clock.h',
+        'test/statistics_delta_reader.cc',
+        'test/statistics_delta_reader.h',
         'test/task_runner_test_template.cc',
         'test/task_runner_test_template.h',
         'test/test_file_util.cc',
diff --git a/base/metrics/statistics_recorder.h b/base/metrics/statistics_recorder.h
index 0716e80..5e53f274 100644
--- a/base/metrics/statistics_recorder.h
+++ b/base/metrics/statistics_recorder.h
@@ -70,9 +70,9 @@ class BASE_EXPORT StatisticsRecorder {
   static HistogramBase* FindHistogram(const std::string& name);
 
   // GetSnapshot copies some of the pointers to registered histograms into the
-  // caller supplied vector (Histograms).  Only histograms with names matching
-  // query are returned. The query must be a substring of histogram name for its
-  // pointer to be copied.
+  // caller supplied vector (Histograms). Only histograms which have |query| as
+  // a substring are copied (an empty string will process all registered
+  // histograms).
   static void GetSnapshot(const std::string& query, Histograms* snapshot);
 
  private:
@@ -88,6 +88,7 @@ class BASE_EXPORT StatisticsRecorder {
   friend class HistogramBaseTest;
   friend class HistogramTest;
   friend class SparseHistogramTest;
+  friend class StatisticsDeltaReaderTest;
   friend class StatisticsRecorderTest;
   FRIEND_TEST_ALL_PREFIXES(HistogramDeltaSerializationTest,
                            DeserializeHistogramAndAddSamples);
diff --git a/base/test/statistics_delta_reader.cc b/base/test/statistics_delta_reader.cc
new file mode 100644
index 0000000..9e77b8b
--- /dev/null
+++ b/base/test/statistics_delta_reader.cc
@@ -0,0 +1,41 @@
+// 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/statistics_recorder.h"
+#include "base/stl_util.h"
+
+namespace base {
+
+StatisticsDeltaReader::StatisticsDeltaReader() {
+  // 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
diff --git a/base/test/statistics_delta_reader.h b/base/test/statistics_delta_reader.h
new file mode 100644
index 0000000..c4e4e99
--- /dev/null
+++ b/base/test/statistics_delta_reader.h
@@ -0,0 +1,41 @@
+// 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.
+
+#ifndef BASE_TEST_STATISTICS_DELTA_READER_H_
+#define BASE_TEST_STATISTICS_DELTA_READER_H_
+
+#include <map>
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram_samples.h"
+
+namespace base {
+
+// This class acts as a differential reader for histogram samples, enabling
+// tests to check that metrics were recorded as they should be.
+// Before using this class, StatisticsRecoder must be initialized.
+class StatisticsDeltaReader {
+ public:
+  StatisticsDeltaReader();
+  ~StatisticsDeltaReader();
+
+  // Returns the histogram data accumulated since this instance was created.
+  // Returns NULL if no samples are available.
+  scoped_ptr<HistogramSamples> GetHistogramSamplesSinceCreation(
+      const std::string& histogram_name);
+
+ private:
+  // Used to determine the histogram changes made during this instance's
+  // lifecycle. This instance takes ownership of the samples, which are deleted
+  // when the instance is destroyed.
+  std::map<std::string, HistogramSamples*> original_samples_;
+
+  DISALLOW_COPY_AND_ASSIGN(StatisticsDeltaReader);
+};
+
+}  // namespace base
+
+#endif  // BASE_TEST_STATISTICS_DELTA_READER_H_
diff --git a/base/test/statistics_delta_reader_unittest.cc b/base/test/statistics_delta_reader_unittest.cc
new file mode 100644
index 0000000..acf8d5a
--- /dev/null
+++ b/base/test/statistics_delta_reader_unittest.cc
@@ -0,0 +1,51 @@
+// 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/memory/scoped_ptr.h"
+#include "base/metrics/histogram.h"
+#include "base/metrics/histogram_samples.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/test/statistics_delta_reader.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+
+class StatisticsDeltaReaderTest : public testing::Test {
+ protected:
+  virtual void SetUp() OVERRIDE {
+    // Each test will have a clean state (no Histogram / BucketRanges
+    // registered).
+    statistics_recorder_ = new StatisticsRecorder();
+  }
+
+  virtual void TearDown() OVERRIDE {
+    delete statistics_recorder_;
+    statistics_recorder_ = NULL;
+  }
+
+  StatisticsRecorder* statistics_recorder_;
+};
+
+TEST_F(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_TRUE(samples);
+  EXPECT_EQ(0, samples->TotalCount());
+
+  // 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
diff --git a/chrome/browser/spellchecker/spellcheck_host_metrics_unittest.cc b/chrome/browser/spellchecker/spellcheck_host_metrics_unittest.cc
index e24a9e5..bd434e5 100644
--- a/chrome/browser/spellchecker/spellcheck_host_metrics_unittest.cc
+++ b/chrome/browser/spellchecker/spellcheck_host_metrics_unittest.cc
@@ -7,10 +7,10 @@
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/message_loop/message_loop.h"
-#include "base/metrics/histogram.h"
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/test/statistics_delta_reader.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 #if defined(OS_WIN)
@@ -18,17 +18,16 @@
 #include "base/win/windows_version.h"
 #endif
 
-using base::HistogramBase;
-using base::HistogramSamples;
-using base::StatisticsRecorder;
-
 class SpellcheckHostMetricsTest : public testing::Test {
  public:
   SpellcheckHostMetricsTest() : loop_(base::MessageLoop::TYPE_DEFAULT) {
   }
 
-  virtual void SetUp() OVERRIDE {
+  static void SetUpTestCase() {
     base::StatisticsRecorder::Initialize();
+  }
+
+  virtual void SetUp() OVERRIDE {
     metrics_.reset(new SpellCheckHostMetrics);
   }
 
@@ -41,32 +40,22 @@ class SpellcheckHostMetricsTest : public testing::Test {
 };
 
 TEST_F(SpellcheckHostMetricsTest, RecordEnabledStats) {
-  scoped_ptr<HistogramSamples> baseline;
-  HistogramBase* histogram =
-      StatisticsRecorder::FindHistogram("SpellCheck.Enabled");
-  if (histogram)
-    baseline = histogram->SnapshotSamples();
+  const char kMetricName[] = "SpellCheck.Enabled";
+  base::StatisticsDeltaReader statistics_delta_reader1;
 
   metrics()->RecordEnabledStats(false);
 
-  histogram =
-      StatisticsRecorder::FindHistogram("SpellCheck.Enabled");
-  ASSERT_TRUE(histogram != NULL);
-  scoped_ptr<HistogramSamples> samples(histogram->SnapshotSamples());
-  if (baseline.get())
-    samples->Subtract(*baseline);
+  scoped_ptr<base::HistogramSamples> samples(
+      statistics_delta_reader1.GetHistogramSamplesSinceCreation(kMetricName));
   EXPECT_EQ(1, samples->GetCount(0));
   EXPECT_EQ(0, samples->GetCount(1));
 
-  baseline.reset(samples.release());
+  base::StatisticsDeltaReader statistics_delta_reader2;
 
   metrics()->RecordEnabledStats(true);
 
-  histogram =
-      StatisticsRecorder::FindHistogram("SpellCheck.Enabled");
-  ASSERT_TRUE(histogram != NULL);
-  samples = histogram->SnapshotSamples();
-  samples->Subtract(*baseline);
+  samples =
+      statistics_delta_reader2.GetHistogramSamplesSinceCreation(kMetricName);
   EXPECT_EQ(0, samples->GetCount(0));
   EXPECT_EQ(1, samples->GetCount(1));
 }
@@ -81,21 +70,16 @@ TEST_F(SpellcheckHostMetricsTest, CustomWordStats) {
 
   // Determine if test failures are due the statistics recorder not being
   // available or because the histogram just isn't there: crbug.com/230534.
-  EXPECT_TRUE(StatisticsRecorder::IsActive());
+  EXPECT_TRUE(base::StatisticsRecorder::IsActive());
 
-  HistogramBase* histogram =
-      StatisticsRecorder::FindHistogram("SpellCheck.CustomWords");
-  ASSERT_TRUE(histogram != NULL);
-  scoped_ptr<HistogramSamples> baseline = histogram->SnapshotSamples();
+  base::StatisticsDeltaReader statistics_delta_reader;
 
   SpellCheckHostMetrics::RecordCustomWordCountStats(23);
-  histogram =
-      StatisticsRecorder::FindHistogram("SpellCheck.CustomWords");
-  ASSERT_TRUE(histogram != NULL);
-  scoped_ptr<HistogramSamples> samples = histogram->SnapshotSamples();
 
-  samples->Subtract(*baseline);
-  EXPECT_EQ(23,samples->sum());
+  scoped_ptr<base::HistogramSamples> samples(
+      statistics_delta_reader.GetHistogramSamplesSinceCreation(
+          "SpellCheck.CustomWords"));
+  EXPECT_EQ(23, samples->sum());
 }
 
 TEST_F(SpellcheckHostMetricsTest, RecordWordCountsDiscardsDuplicates) {
@@ -113,59 +97,38 @@ TEST_F(SpellcheckHostMetricsTest, RecordWordCountsDiscardsDuplicates) {
   metrics()->RecordCheckedWordStats(base::ASCIIToUTF16("test"), false);
   RecordWordCountsForTesting();
 
-  // Get baselines for all affected histograms.
-  scoped_ptr<HistogramSamples> baselines[arraysize(histogramName)];
-  for (size_t i = 0; i < arraysize(histogramName); ++i) {
-    HistogramBase* histogram =
-        StatisticsRecorder::FindHistogram(histogramName[i]);
-    if (histogram)
-      baselines[i] = histogram->SnapshotSamples();
-  }
+  // Start the reader.
+  base::StatisticsDeltaReader statistics_delta_reader;
 
   // Nothing changed, so this invocation should not affect any histograms.
   RecordWordCountsForTesting();
 
   // Get samples for all affected histograms.
-  scoped_ptr<HistogramSamples> samples[arraysize(histogramName)];
+  scoped_ptr<base::HistogramSamples> samples;
   for (size_t i = 0; i < arraysize(histogramName); ++i) {
-    HistogramBase* histogram =
-        StatisticsRecorder::FindHistogram(histogramName[i]);
-    ASSERT_TRUE(histogram != NULL);
-    samples[i] = histogram->SnapshotSamples();
-    if (baselines[i].get())
-      samples[i]->Subtract(*baselines[i]);
-
-    EXPECT_EQ(0, samples[i]->TotalCount());
+    samples = statistics_delta_reader.GetHistogramSamplesSinceCreation(
+        histogramName[i]);
+    EXPECT_EQ(0, samples->TotalCount());
   }
 }
 
 TEST_F(SpellcheckHostMetricsTest, RecordSpellingServiceStats) {
   const char kMetricName[] = "SpellCheck.SpellingService.Enabled";
-  scoped_ptr<HistogramSamples> baseline;
-  HistogramBase* histogram = StatisticsRecorder::FindHistogram(kMetricName);
-  if (histogram)
-    baseline = histogram->SnapshotSamples();
+  base::StatisticsDeltaReader statistics_delta_reader1;
 
   metrics()->RecordSpellingServiceStats(false);
 
-  histogram =
-      StatisticsRecorder::FindHistogram(kMetricName);
-  ASSERT_TRUE(histogram != NULL);
-  scoped_ptr<HistogramSamples> samples(histogram->SnapshotSamples());
-  if (baseline.get())
-    samples->Subtract(*baseline);
+  scoped_ptr<base::HistogramSamples> samples(
+      statistics_delta_reader1.GetHistogramSamplesSinceCreation(kMetricName));
   EXPECT_EQ(1, samples->GetCount(0));
   EXPECT_EQ(0, samples->GetCount(1));
 
-  baseline.reset(samples.release());
+  base::StatisticsDeltaReader statistics_delta_reader2;
 
   metrics()->RecordSpellingServiceStats(true);
 
-  histogram =
-      StatisticsRecorder::FindHistogram(kMetricName);
-  ASSERT_TRUE(histogram != NULL);
-  samples = histogram->SnapshotSamples();
-  samples->Subtract(*baseline);
+  samples =
+      statistics_delta_reader2.GetHistogramSamplesSinceCreation(kMetricName);
   EXPECT_EQ(0, samples->GetCount(0));
   EXPECT_EQ(1, samples->GetCount(1));
 }
diff --git a/chrome/browser/ui/cocoa/browser/password_generation_bubble_controller_unittest.mm b/chrome/browser/ui/cocoa/browser/password_generation_bubble_controller_unittest.mm
index c3211714..0637f24 100644
--- a/chrome/browser/ui/cocoa/browser/password_generation_bubble_controller_unittest.mm
+++ b/chrome/browser/ui/cocoa/browser/password_generation_bubble_controller_unittest.mm
@@ -5,19 +5,15 @@
 #import "chrome/browser/ui/cocoa/browser/password_generation_bubble_controller.h"
 
 #include "base/logging.h"
-#include "base/metrics/histogram.h"
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/strings/sys_string_conversions.h"
+#include "base/test/statistics_delta_reader.h"
 #include "chrome/browser/ui/cocoa/cocoa_profile_test.h"
 #include "components/autofill/core/browser/password_generator.h"
 #include "components/autofill/core/common/password_form.h"
 #include "testing/gtest_mac.h"
 
-using base::HistogramBase;
-using base::HistogramSamples;
-using base::StatisticsRecorder;
-
 const char kHistogramName[] = "PasswordGeneration.UserActions";
 
 class PasswordGenerationBubbleControllerTest : public CocoaProfileTest {
@@ -26,7 +22,7 @@ class PasswordGenerationBubbleControllerTest : public CocoaProfileTest {
       : controller_(nil) {}
 
   static void SetUpTestCase() {
-    StatisticsRecorder::Initialize();
+    base::StatisticsRecorder::Initialize();
   }
 
   virtual void SetUp() {
@@ -34,11 +30,6 @@ class PasswordGenerationBubbleControllerTest : public CocoaProfileTest {
 
     generator_.reset(new autofill::PasswordGenerator(20));
 
-    HistogramBase* histogram =
-        StatisticsRecorder::FindHistogram(kHistogramName);
-    if (histogram)
-      original_ = histogram->SnapshotSamples();
-
     SetUpController();
   }
 
@@ -65,27 +56,10 @@ class PasswordGenerationBubbleControllerTest : public CocoaProfileTest {
     controller_ = nil;
   }
 
-  HistogramSamples* GetHistogramSamples() {
-    HistogramBase* histogram =
-        StatisticsRecorder::FindHistogram(kHistogramName);
-    if (histogram) {
-      current_ = histogram->SnapshotSamples();
-      if (original_.get())
-        current_->Subtract(*original_.get());
-    }
-    return current_.get();
-  }
-
  protected:
   // Weak.
   PasswordGenerationBubbleController* controller_;
 
-  // Used to determine the histogram changes made just for this specific
-  // test run.
-  scoped_ptr<HistogramSamples> original_;
-
-  scoped_ptr<HistogramSamples> current_;
-
   scoped_ptr<autofill::PasswordGenerator> generator_;
 };
 
@@ -107,12 +81,14 @@ TEST_F(PasswordGenerationBubbleControllerTest, Regenerate) {
 }
 
 TEST_F(PasswordGenerationBubbleControllerTest, UMALogging) {
+  base::StatisticsDeltaReader statistics_delta_reader;
   [controller() showWindow:nil];
 
   // Do nothing.
   CloseController();
 
-  HistogramSamples* samples = GetHistogramSamples();
+  scoped_ptr<base::HistogramSamples> samples(
+      statistics_delta_reader.GetHistogramSamplesSinceCreation(kHistogramName));
   EXPECT_EQ(
       1,
       samples->GetCount(autofill::password_generation::IGNORE_FEATURE));
@@ -131,7 +107,8 @@ TEST_F(PasswordGenerationBubbleControllerTest, UMALogging) {
   [controller() fillPassword:nil];
   CloseController();
 
-  samples = GetHistogramSamples();
+  samples =
+      statistics_delta_reader.GetHistogramSamplesSinceCreation(kHistogramName);
   EXPECT_EQ(
       1,
       samples->GetCount(autofill::password_generation::IGNORE_FEATURE));
@@ -149,7 +126,8 @@ TEST_F(PasswordGenerationBubbleControllerTest, UMALogging) {
   [controller() fillPassword:nil];
   CloseController();
 
-  samples = GetHistogramSamples();
+  samples =
+      statistics_delta_reader.GetHistogramSamplesSinceCreation(kHistogramName);
   EXPECT_EQ(
       1,
       samples->GetCount(autofill::password_generation::IGNORE_FEATURE));
@@ -160,5 +138,4 @@ TEST_F(PasswordGenerationBubbleControllerTest, UMALogging) {
       1,
       samples->GetCount(
           autofill::password_generation::ACCEPT_ORIGINAL_PASSWORD));
-
 }
diff --git a/net/url_request/url_request_throttler_unittest.cc b/net/url_request/url_request_throttler_unittest.cc
index b964b10..ebc917f 100644
--- a/net/url_request/url_request_throttler_unittest.cc
+++ b/net/url_request/url_request_throttler_unittest.cc
@@ -5,13 +5,13 @@
 #include "net/url_request/url_request_throttler_manager.h"
 
 #include "base/memory/scoped_ptr.h"
-#include "base/metrics/histogram.h"
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/pickle.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
+#include "base/test/statistics_delta_reader.h"
 #include "base/time/time.h"
 #include "net/base/load_flags.h"
 #include "net/base/request_priority.h"
@@ -29,10 +29,7 @@ namespace net {
 
 namespace {
 
-using base::Histogram;
-using base::HistogramBase;
-using base::HistogramSamples;
-using base::StatisticsRecorder;
+const char kRequestThrottledHistogramName[] = "Throttling.RequestThrottled";
 
 class MockURLRequestThrottlerEntry : public URLRequestThrottlerEntry {
  public:
@@ -176,76 +173,26 @@ class URLRequestThrottlerEntryTest : public testing::Test {
   URLRequestThrottlerEntryTest()
       : request_(GURL(), DEFAULT_PRIORITY, NULL, &context_) {}
 
-  virtual void SetUp();
-  virtual void TearDown();
+  static void SetUpTestCase() {
+    base::StatisticsRecorder::Initialize();
+  }
 
-  // After calling this function, histogram snapshots in |samples_| contain
-  // only the delta caused by the test case currently running.
-  void CalculateHistogramDeltas();
+  virtual void SetUp();
 
   TimeTicks now_;
   MockURLRequestThrottlerManager manager_;  // Dummy object, not used.
   scoped_refptr<MockURLRequestThrottlerEntry> entry_;
 
-  std::map<std::string, HistogramSamples*> original_samples_;
-  std::map<std::string, HistogramSamples*> samples_;
-
   TestURLRequestContext context_;
   TestURLRequest request_;
 };
 
-// List of all histograms we care about in these unit tests.
-const char* kHistogramNames[] = {
-  "Throttling.FailureCountAtSuccess",
-  "Throttling.PerceivedDowntime",
-  "Throttling.RequestThrottled",
-  "Throttling.SiteOptedOut",
-};
-
 void URLRequestThrottlerEntryTest::SetUp() {
   request_.SetLoadFlags(0);
 
   now_ = TimeTicks::Now();
   entry_ = new MockURLRequestThrottlerEntry(&manager_);
   entry_->ResetToBlank(now_);
-
-  for (size_t i = 0; i < arraysize(kHistogramNames); ++i) {
-    // Must retrieve original samples for each histogram for comparison
-    // as other tests may affect them.
-    const char* name = kHistogramNames[i];
-    HistogramBase* histogram = StatisticsRecorder::FindHistogram(name);
-    if (histogram) {
-      original_samples_[name] = histogram->SnapshotSamples().release();
-    } else {
-      original_samples_[name] = NULL;
-    }
-  }
-}
-
-void URLRequestThrottlerEntryTest::TearDown() {
-  STLDeleteValues(&original_samples_);
-  STLDeleteValues(&samples_);
-}
-
-void URLRequestThrottlerEntryTest::CalculateHistogramDeltas() {
-  for (size_t i = 0; i < arraysize(kHistogramNames); ++i) {
-    const char* name = kHistogramNames[i];
-    HistogramSamples* original = original_samples_[name];
-
-    HistogramBase* histogram = StatisticsRecorder::FindHistogram(name);
-    if (histogram) {
-      ASSERT_EQ(HistogramBase::kUmaTargetedHistogramFlag, histogram->flags());
-
-      scoped_ptr<HistogramSamples> samples(histogram->SnapshotSamples());
-      if (original)
-        samples->Subtract(*original);
-      samples_[name] = samples.release();
-    }
-  }
-
-  // Ensure we don't accidentally use the originals in our tests.
-  STLDeleteValues(&original_samples_);
-  original_samples_.clear();
 }
 
 std::ostream& operator<<(std::ostream& out, const base::TimeTicks& time) {
@@ -265,6 +212,7 @@ TEST_F(URLRequestThrottlerEntryTest, CanThrottleRequest) {
 }
 
 TEST_F(URLRequestThrottlerEntryTest, InterfaceDuringExponentialBackoff) {
+  base::StatisticsDeltaReader statistics_delta_reader;
   entry_->set_exponential_backoff_release_time(
       entry_->fake_time_now_ + TimeDelta::FromMilliseconds(1));
   EXPECT_TRUE(entry_->ShouldRejectRequest(request_));
@@ -273,21 +221,26 @@ TEST_F(URLRequestThrottlerEntryTest, InterfaceDuringExponentialBackoff) {
   request_.SetLoadFlags(LOAD_MAYBE_USER_GESTURE);
   EXPECT_FALSE(entry_->ShouldRejectRequest(request_));
 
-  CalculateHistogramDeltas();
-  ASSERT_EQ(1, samples_["Throttling.RequestThrottled"]->GetCount(0));
-  ASSERT_EQ(1, samples_["Throttling.RequestThrottled"]->GetCount(1));
+  scoped_ptr<base::HistogramSamples> samples(
+      statistics_delta_reader.GetHistogramSamplesSinceCreation(
+          kRequestThrottledHistogramName));
+  ASSERT_EQ(1, samples->GetCount(0));
+  ASSERT_EQ(1, samples->GetCount(1));
 }
 
 TEST_F(URLRequestThrottlerEntryTest, InterfaceNotDuringExponentialBackoff) {
+  base::StatisticsDeltaReader statistics_delta_reader;
   entry_->set_exponential_backoff_release_time(entry_->fake_time_now_);
   EXPECT_FALSE(entry_->ShouldRejectRequest(request_));
   entry_->set_exponential_backoff_release_time(
       entry_->fake_time_now_ - TimeDelta::FromMilliseconds(1));
   EXPECT_FALSE(entry_->ShouldRejectRequest(request_));
 
-  CalculateHistogramDeltas();
-  ASSERT_EQ(2, samples_["Throttling.RequestThrottled"]->GetCount(0));
-  ASSERT_EQ(0, samples_["Throttling.RequestThrottled"]->GetCount(1));
+  scoped_ptr<base::HistogramSamples> samples(
+      statistics_delta_reader.GetHistogramSamplesSinceCreation(
+          kRequestThrottledHistogramName));
+  ASSERT_EQ(2, samples->GetCount(0));
+  ASSERT_EQ(0, samples->GetCount(1));
 }
 
 TEST_F(URLRequestThrottlerEntryTest, InterfaceUpdateFailure) {