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

base/metrics/histogram_base_unittest.cc

 // Copyright (c) 2012 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 <vector>
 
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/sparse_histogram.h"
 #include "base/metrics/statistics_recorder.h"
@@ skipping 11 matching lines @@
   }
 
   ~HistogramBaseTest() override {
     HistogramBase::report_histogram_ = nullptr;
   }
 
   void ResetStatisticsRecorder() {
     // It is necessary to fully destruct any existing StatisticsRecorder
     // before creating a new one.
     statistics_recorder_.reset();
-    statistics_recorder_.reset(new StatisticsRecorder());
+    statistics_recorder_ = StatisticsRecorder::CreateTemporaryForTesting();
   }
 
   HistogramBase* GetCreationReportHistogram(const std::string& name) {
     HistogramBase::EnableActivityReportHistogram(name);
     return HistogramBase::report_histogram_;
   }
 
  private:
   std::unique_ptr<StatisticsRecorder> statistics_recorder_;
 
@@ skipping 170 matching lines @@
   BooleanHistogram::FactoryGet("CRH-Boolean", 0);
   CustomHistogram::FactoryGet("CRH-Custom", ranges, 0);
   SparseHistogram::FactoryGet("CRH-Sparse", 0);
   samples = report->SnapshotSamples();
   EXPECT_EQ(1, samples->GetCount(HISTOGRAM_REPORT_CREATED));
   EXPECT_EQ(9, samples->GetCount(HISTOGRAM_REPORT_HISTOGRAM_CREATED));
   EXPECT_EQ(5, samples->GetCount(HISTOGRAM_REPORT_HISTOGRAM_LOOKUP));
 }
 
 }  // namespace base