make debug mode ~20% faster on linux

base/threading/thread_checker_impl.cc

 // 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.
 
 #include "base/threading/thread_checker_impl.h"
 
 namespace base {
 
 ThreadCheckerImpl::ThreadCheckerImpl()
-    : valid_thread_id_(kInvalidThreadId) {
+    : valid_thread_id_() {
   EnsureThreadIdAssigned();
 }
 
 ThreadCheckerImpl::~ThreadCheckerImpl() {}
 
 bool ThreadCheckerImpl::CalledOnValidThread() const {
   EnsureThreadIdAssigned();
   AutoLock auto_lock(lock_);
-  return valid_thread_id_ == PlatformThread::CurrentId();
+  return valid_thread_id_ == PlatformThread::CurrentRef();
 }
 
 void ThreadCheckerImpl::DetachFromThread() {
   AutoLock auto_lock(lock_);
-  valid_thread_id_ = kInvalidThreadId;
+  valid_thread_id_ = PlatformThreadRef();
 }
 
 void ThreadCheckerImpl::EnsureThreadIdAssigned() const {
   AutoLock auto_lock(lock_);
-  if (valid_thread_id_ != kInvalidThreadId)
-    return;
-  valid_thread_id_ = PlatformThread::CurrentId();
+  if (valid_thread_id_.is_null()) {
+    valid_thread_id_ = PlatformThread::CurrentRef();
+  }
 }
 
 }  // namespace base