[ObjC ARC] Converts ios/chrome/browser/web:web to ARC.

ios/chrome/browser/web/dom_altering_lock.mm

 // Copyright 2014 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/web/dom_altering_lock.h"
 
 #include "base/logging.h"
 #include "ios/web/public/web_thread.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 DEFINE_WEB_STATE_USER_DATA_KEY(DOMAlteringLock);
 
 DOMAlteringLock::DOMAlteringLock(web::WebState* web_state) {
 }
 
 DOMAlteringLock::~DOMAlteringLock() {
 }
 
 void DOMAlteringLock::Acquire(id<DOMAltering> feature,
                               ProceduralBlockWithBool lockAction) {
@@ skipping 19 matching lines @@
   current_dom_altering_feature_.reset(feature);
   lockAction(YES);
 }
 
 // Release the lock on the DOM tree.
 void DOMAlteringLock::Release(id<DOMAltering> feature) {
   DCHECK_CURRENTLY_ON(web::WebThread::UI);
   if (current_dom_altering_feature_.get() == feature)
     current_dom_altering_feature_.reset();
 }