Remove all but one use of WeakPtrFactory::DetachFromThread.

base/memory/weak_ptr.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/memory/weak_ptr.h"
 
 namespace base {
 namespace internal {
 
 WeakReference::Flag::Flag() : is_valid_(true) {
+  // Flags only become bound when checked for validity, or invalidated,
+  // so that we can check that later validity/invalidation operations on
+  // the same Flag take place on the same thread.
+  thread_checker_.DetachFromThread();
 }
 
 void WeakReference::Flag::Invalidate() {
   // The flag being invalidated with a single ref implies that there are no
   // weak pointers in existence. Allow deletion on other thread in this case.
-  DCHECK(thread_checker_.CalledOnValidThread() || HasOneRef());
+  DCHECK(thread_checker_.CalledOnValidThread() || HasOneRef())
+      << "WeakPtrs must be checked and invalidated on the same thread.";
   is_valid_ = false;
 }
 
 bool WeakReference::Flag::IsValid() const {
-  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(thread_checker_.CalledOnValidThread())
+      << "WeakPtrs must be checked and invalidated on the same thread.";
   return is_valid_;
 }
 
 WeakReference::Flag::~Flag() {
 }
 
 WeakReference::WeakReference() {
 }
 
 WeakReference::WeakReference(const Flag* flag) : flag_(flag) {
 }
 
 WeakReference::~WeakReference() {
 }
 
 bool WeakReference::is_valid() const {
   return flag_ && flag_->IsValid();
 }
 
 WeakReferenceOwner::WeakReferenceOwner() {
 }
 
 WeakReferenceOwner::~WeakReferenceOwner() {
   Invalidate();
 }
 
 WeakReference WeakReferenceOwner::GetRef() const {
-  // We also want to reattach to the current thread if all previous references
-  // have gone away.
+  // If we hold the last reference to the Flag then create a new one.
   if (!HasRefs())
     flag_ = new WeakReference::Flag();
+
   return WeakReference(flag_);
 }
 
 void WeakReferenceOwner::Invalidate() {
   if (flag_) {
     flag_->Invalidate();
     flag_ = NULL;
   }
 }
 
 WeakPtrBase::WeakPtrBase() {
 }
 
 WeakPtrBase::~WeakPtrBase() {
 }
 
 WeakPtrBase::WeakPtrBase(const WeakReference& ref) : ref_(ref) {
 }
 
 }  // namespace internal
 }  // namespace base