Create ForTesting method rather than declaring a dozen "friend" classes.

base/metrics/histogram_delta_serialization_unittest.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/metrics/histogram_delta_serialization.h"
 
 #include <vector>
 
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/statistics_recorder.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace base {
 
 TEST(HistogramDeltaSerializationTest, DeserializeHistogramAndAddSamples) {
-  StatisticsRecorder statistic_recorder;
+  std::unique_ptr<StatisticsRecorder> statistic_recorder(
+      StatisticsRecorder::CreateTemporaryForTesting());
   HistogramDeltaSerialization serializer("HistogramDeltaSerializationTest");
   std::vector<std::string> deltas;
   // Nothing was changed yet.
   serializer.PrepareAndSerializeDeltas(&deltas, true);
   EXPECT_TRUE(deltas.empty());
 
   HistogramBase* histogram = Histogram::FactoryGet(
       "TestHistogram", 1, 1000, 10, HistogramBase::kIPCSerializationSourceFlag);
   histogram->Add(1);
   histogram->Add(10);
@@ skipping 17 matching lines @@
   HistogramDeltaSerialization::DeserializeAndAddSamples(deltas);
 
   std::unique_ptr<HistogramSamples> snapshot2(histogram->SnapshotSamples());
   EXPECT_EQ(2, snapshot2->GetCount(1));
   EXPECT_EQ(2, snapshot2->GetCount(10));
   EXPECT_EQ(2, snapshot2->GetCount(100));
   EXPECT_EQ(2, snapshot2->GetCount(1000));
 }
 
 }  // namespace base