Use lock-free lazy initialization for static histogram references

net/disk_cache/stats_histogram.h

Index: net/disk_cache/stats_histogram.h
===================================================================
--- net/disk_cache/stats_histogram.h	(revision 79771)
+++ net/disk_cache/stats_histogram.h	(working copy)
@@ -35,8 +35,7 @@
       : Histogram(name, minimum, maximum, bucket_count), init_(false) {}
   ~StatsHistogram();
 
-  static scoped_refptr<StatsHistogram>
-      StatsHistogramFactoryGet(const std::string& name);
+  static StatsHistogram* StatsHistogramFactoryGet(const std::string& name);
 
   // We'll be reporting data from the given set of cache stats.
   bool Init(const Stats* stats);