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

chrome/browser/metrics/subprocess_metrics_provider_unittest.cc

 // Copyright 2016 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 "chrome/browser/metrics/subprocess_metrics_provider.h"
 
 #include <memory>
 
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram.h"
@@ skipping 49 matching lines @@
     // Get this first so it isn't created inside a persistent allocator.
     base::PersistentHistogramAllocator::GetCreateHistogramResultHistogram();
 
     // MergeHistogramDeltas needs to be called beause it uses a histogram
     // macro which caches a pointer to a histogram. If not done before setting
     // a persistent global allocator, then it would point into memory that
     // will go away.
     provider_.MergeHistogramDeltas();
 
     // Create a dedicated StatisticsRecorder for this test.
-    test_recorder_.reset(new base::StatisticsRecorder());
+    test_recorder_ = base::StatisticsRecorder::CreateTemporaryForTesting();
 
     // Create a global allocator using a block of memory from the heap.
     base::GlobalHistogramAllocator::CreateWithLocalMemory(TEST_MEMORY_SIZE,
                                                           0, "");
   }
 
   ~SubprocessMetricsProviderTest() override {
     base::GlobalHistogramAllocator::ReleaseForTesting();
   }
 
@@ skipping 73 matching lines @@
   foo->Add(10);
   bar->Add(20);
   DeregisterSubprocessAllocator(123);
   EXPECT_EQ(2U, GetSnapshotHistogramCount());
 
   // Further snapshots should be empty even if things have changed.
   foo->Add(10);
   bar->Add(20);
   EXPECT_EQ(0U, GetSnapshotHistogramCount());
 }