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

base/metrics/statistics_recorder.cc

Index: base/metrics/statistics_recorder.cc
diff --git a/base/metrics/statistics_recorder.cc b/base/metrics/statistics_recorder.cc
index 4eecddade8058c723392abb32152778e3c5c59eb..151955ab7333fada66ca335cdb03b7fd836c18b8 100644
--- a/base/metrics/statistics_recorder.cc
+++ b/base/metrics/statistics_recorder.cc
@@ -10,6 +10,7 @@
 #include "base/debug/leak_annotations.h"
 #include "base/json/string_escape.h"
 #include "base/logging.h"
+#include "base/memory/ptr_util.h"
 #include "base/metrics/histogram.h"
 #include "base/metrics/metrics_hashes.h"
 #include "base/metrics/persistent_histogram_allocator.h"
@@ -421,6 +422,12 @@ void StatisticsRecorder::ForgetHistogramForTesting(base::StringPiece name) {
 }
 
 // static
+std::unique_ptr<StatisticsRecorder>
+StatisticsRecorder::CreateTemporaryForTesting() {
+  return WrapUnique(new StatisticsRecorder());
+}
+
+// static
 void StatisticsRecorder::UninitializeForTesting() {
   // Stop now if it's never been initialized.
   if (lock_ == NULL || histograms_ == NULL)