Revert r21117 as it caused reliability failures....

chrome/common/histogram_synchronizer.cc

 // Copyright (c) 2006-2008 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.
 
 #include <string>
 
 #include "base/histogram.h"
 #include "base/logging.h"
 #include "base/string_util.h"
-#include "base/thread.h"
 #include "chrome/browser/browser.h"
 #include "chrome/browser/browser_list.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/renderer_host/render_process_host.h"
 #include "chrome/common/child_thread.h"
 #include "chrome/common/render_messages.h"
 #include "chrome/common/histogram_synchronizer.h"
 
 using base::Time;
 using base::TimeDelta;
@@ skipping 231 matching lines @@
 
 bool HistogramSynchronizer::IsOnIoThread() {
   if (io_message_loop_ == NULL) {
     io_message_loop_ = MessageLoop::current();
   }
   return (MessageLoop::current() == io_message_loop_);
 }
 
 // static
 HistogramSynchronizer* HistogramSynchronizer::histogram_synchronizer_ = NULL;