Revert of [ObjC ARC] Converts ios/chrome/browser/prefs:prefs to ARC.Automatically generated ARC...

ios/chrome/browser/prefs/pref_observer_bridge.mm

 // Copyright 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.
 
 #import "ios/chrome/browser/prefs/pref_observer_bridge.h"
 
 #include "base/bind.h"
 #include "components/prefs/pref_change_registrar.h"
 
-#if !defined(__has_feature) || !__has_feature(objc_arc)
-#error "This file requires ARC support."
-#endif
-
 PrefObserverBridge::PrefObserverBridge(id<PrefObserverDelegate> delegate)
     : delegate_(delegate) {
 }
 
 PrefObserverBridge::~PrefObserverBridge() {
 }
 
 void PrefObserverBridge::ObserveChangesForPreference(
     const std::string& pref_name,
     PrefChangeRegistrar* registrar) {
   PrefChangeRegistrar::NamedChangeCallback callback = base::Bind(
       &PrefObserverBridge::OnPreferenceChanged, base::Unretained(this));
   registrar->Add(pref_name.c_str(), callback);
 }
 
 void PrefObserverBridge::OnPreferenceChanged(const std::string& pref_name) {
   [delegate_ onPreferenceChanged:pref_name];
 }