[ObjC ARC] Converts components/signin/ios/browser:browser to ARC.

components/signin/ios/browser/merge_session_observer_bridge.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.
 
 #include "components/signin/ios/browser/merge_session_observer_bridge.h"
 
 #include "base/logging.h"
 #include "google_apis/gaia/google_service_auth_error.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 MergeSessionObserverBridge::MergeSessionObserverBridge(
     id<MergeSessionObserverBridgeDelegate> delegate,
     GaiaCookieManagerService* cookie_manager_service)
     : delegate_(delegate), cookie_manager_service_(cookie_manager_service) {
   DCHECK(delegate);
   DCHECK(cookie_manager_service);
   cookie_manager_service_->AddObserver(this);
 }
 
 MergeSessionObserverBridge::~MergeSessionObserverBridge() {
   cookie_manager_service_->RemoveObserver(this);
 }
 
 void MergeSessionObserverBridge::OnAddAccountToCookieCompleted(
     const std::string& account_id,
     const GoogleServiceAuthError& error) {
   [delegate_ onMergeSessionCompleted:account_id error:error];
 }