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

base/metrics/statistics_recorder.h

Index: base/metrics/statistics_recorder.h
diff --git a/base/metrics/statistics_recorder.h b/base/metrics/statistics_recorder.h
index f40bb4e5495fcaa21fbdc59b9b22d65751ec9e29..002758e840e1aa404409e6a14785615e790c9ce5 100644
--- a/base/metrics/statistics_recorder.h
+++ b/base/metrics/statistics_recorder.h
@@ -172,7 +172,15 @@ class BASE_EXPORT StatisticsRecorder {
   // memory is being released.
   static void ForgetHistogramForTesting(base::StringPiece name);
 
-  // Reset any global instance of the statistics-recorder that was created
+  // Creates a local StatisticsRecorder object for testing purposes. All new
+  // histograms will be registered in it until it is destructed or pushed
+  // aside for the lifetime of yet another SR object. The destruction of the
+  // returned object will re-activate the previous one. Always release SR
+  // objects in the opposite order to which they're created.
+  static std::unique_ptr<StatisticsRecorder> CreateTemporaryForTesting()
+      WARN_UNUSED_RESULT;
+
+  // Resets any global instance of the statistics-recorder that was created
   // by a call to Initialize().
   static void UninitializeForTesting();
 
@@ -187,17 +195,6 @@ class BASE_EXPORT StatisticsRecorder {
   typedef std::map<uint32_t, std::list<const BucketRanges*>*> RangesMap;
 
   friend struct DefaultLazyInstanceTraits<StatisticsRecorder>;
-  friend class ::SubprocessMetricsProviderTest;
-  friend class HistogramBaseTest;
-  friend class HistogramSnapshotManagerTest;
-  friend class HistogramTest;
-  friend class JsonPrefStoreTest;
-  friend class PersistentHistogramAllocatorTest;
-  friend class SharedHistogramTest;
-  friend class SparseHistogramTest;
-  friend class StatisticsRecorderTest;
-  FRIEND_TEST_ALL_PREFIXES(HistogramDeltaSerializationTest,
-                           DeserializeHistogramAndAddSamples);
 
   // Imports histograms from global persistent memory. The global lock must
   // not be held during this call.