Use lock-free lazy initialization for static histogram references

chrome/browser/metrics/thread_watcher.h

 // Copyright (c) 2011 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.
 
 // This file defines a WatchDog thread that monitors the responsiveness of other
 // browser threads like UI, IO, DB, FILE and CACHED threads. It also defines
 // ThreadWatcher class which performs health check on threads that would like to
 // be watched. This file also defines ThreadWatcherList class that has list of
 // all active ThreadWatcher objects.
 //
@@ skipping 168 matching lines @@
   // This is set to true if thread watcher is watching.
   bool active_;
 
   // The counter tracks least number of ping messages that will be sent to
   // watched thread before the ping-pong mechanism will go into an extended
   // sleep. If this value is zero, then the mechanism is in an extended sleep,
   // and awaiting some observed user action before continuing.
   int ping_count_;
 
   // Histogram that keeps track of response times for the watched thread.
-  scoped_refptr<base::Histogram> histogram_;
+  base::Histogram* histogram_;
 
   // We use this factory to create callback tasks for ThreadWatcher object. We
   // use this during ping-pong messaging between WatchDog thread and watched
   // thread.
   ScopedRunnableMethodFactory<ThreadWatcher> method_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadWatcher);
 };
 
 // Class with a list of all active thread watchers.  A thread watcher is active
@@ skipping 119 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(WatchDogThread);
 };
 
 // DISABLE_RUNNABLE_METHOD_REFCOUNT is a convenience macro for disabling
 // refcounting of ThreadWatcher and ThreadWatcherList classes.
 DISABLE_RUNNABLE_METHOD_REFCOUNT(ThreadWatcher);
 DISABLE_RUNNABLE_METHOD_REFCOUNT(ThreadWatcherList);
 
 #endif  // CHROME_BROWSER_METRICS_THREAD_WATCHER_H_