Use lock-free lazy initialization for static histogram references

base/message_loop.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.
 
 #ifndef BASE_MESSAGE_LOOP_H_
 #define BASE_MESSAGE_LOOP_H_
 #pragma once
 
 #include <queue>
 #include <string>
@@ skipping 455 matching lines @@
   ObserverList<DestructionObserver> destruction_observers_;
 
   // A recursion block that prevents accidentally running additonal tasks when
   // insider a (accidentally induced?) nested message pump.
   bool nestable_tasks_allowed_;
 
   bool exception_restoration_;
 
   std::string thread_name_;
   // A profiling histogram showing the counts of various messages and events.
-  scoped_refptr<base::Histogram> message_histogram_;
+  base::Histogram* message_histogram_;
 
   // A null terminated list which creates an incoming_queue of tasks that are
   // acquired under a mutex for processing on this instance's thread. These
   // tasks have not yet been sorted out into items for our work_queue_ vs
   // items that will be handled by the TimerManager.
   TaskQueue incoming_queue_;
   // Protect access to incoming_queue_.
   mutable base::Lock incoming_queue_lock_;
 
   RunState* state_;
@@ skipping 141 matching lines @@
 #endif  // defined(OS_POSIX)
 };
 
 // Do not add any member variables to MessageLoopForIO!  This is important b/c
 // MessageLoopForIO is often allocated via MessageLoop(TYPE_IO).  Any extra
 // data that you need should be stored on the MessageLoop's pump_ instance.
 COMPILE_ASSERT(sizeof(MessageLoop) == sizeof(MessageLoopForIO),
                MessageLoopForIO_should_not_have_extra_member_variables);
 
 #endif  // BASE_MESSAGE_LOOP_H_