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

ios/chrome/browser/callback_counter.mm

 // Copyright 2015 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 "ios/chrome/browser/callback_counter.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 CallbackCounter::CallbackCounter(const FinalCallback& final_callback)
     : callback_count_(0U), final_callback_(final_callback) {
   DCHECK(!final_callback.is_null());
 }
 
 CallbackCounter::~CallbackCounter() {
   DCHECK_EQ(0U, callback_count_);
 }
 
 void CallbackCounter::IncrementCount() {
   DCHECK(thread_checker_.CalledOnValidThread());
   DCHECK(!final_callback_.is_null());
   ++callback_count_;
 }
 
 void CallbackCounter::DecrementCount() {
   DCHECK(thread_checker_.CalledOnValidThread());
   DCHECK(callback_count_);
 
   --callback_count_;
   if (callback_count_ == 0) {
     final_callback_.Run();
     final_callback_.Reset();
   }
 }