Remove usage of base::ObserverList<T>::Iter::GetNext() in //components/prefs.

components/prefs/pref_notifier_impl.cc

 // Copyright (c) 2012 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 "components/prefs/pref_notifier_impl.h"
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "components/prefs/pref_service.h"
 
 PrefNotifierImpl::PrefNotifierImpl() : pref_service_(nullptr) {}
 
 PrefNotifierImpl::PrefNotifierImpl(PrefService* service)
     : pref_service_(service) {
 }
 
 PrefNotifierImpl::~PrefNotifierImpl() {
   DCHECK(thread_checker_.CalledOnValidThread());
 
   // Verify that there are no pref observers when we shut down.
   for (const auto& observer_list : pref_observers_) {
-    PrefObserverList::Iterator obs_iterator(observer_list.second.get());
-    if (obs_iterator.GetNext()) {
+    if (observer_list.second->begin() != observer_list.second->end())

[Pam (message me for reviews), 2016/10/14 20:35:26]

Why not observer_list.second->size() ?

[dcheng, 2016/10/14 20:38:40]

size() isn't exposed by ObserverList. Since ObserverList is thread-safe-ish,
allow reentrancy, and allows mutation of ObserverList while dispatching to
observers, it turns out to be a bit tricky to implement this in a performant
way.

Thus, this somewhat silly looking check.

       LOG(WARNING) << "Pref observer found at shutdown.";
-    }
   }
 
   // Same for initialization observers.
   if (!init_observers_.empty())
     LOG(WARNING) << "Init observer found at shutdown.";
 
   pref_observers_.clear();
   init_observers_.clear();
 }
 
@@ skipping 61 matching lines @@
 
   FOR_EACH_OBSERVER(PrefObserver,
                     *(observer_iterator->second),
                     OnPreferenceChanged(pref_service_, path));
 }
 
 void PrefNotifierImpl::SetPrefService(PrefService* pref_service) {
   DCHECK(pref_service_ == nullptr);
   pref_service_ = pref_service;
 }