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

components/signin/ios/browser/profile_oauth2_token_service_ios_provider.mm

 // Copyright 2016 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/profile_oauth2_token_service_ios_provider.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 std::vector<ProfileOAuth2TokenServiceIOSProvider::AccountInfo>
 ProfileOAuth2TokenServiceIOSProvider::GetAllAccounts() const {
   return std::vector<ProfileOAuth2TokenServiceIOSProvider::AccountInfo>();
 }
 
 void ProfileOAuth2TokenServiceIOSProvider::GetAccessToken(
     const std::string& gaia_id,
     const std::string& client_id,
     const std::string& client_secret,
     const std::set<std::string>& scopes,
     const AccessTokenCallback& callback) {}
 
 AuthenticationErrorCategory
 ProfileOAuth2TokenServiceIOSProvider::GetAuthenticationErrorCategory(
     const std::string& gaia_id,
     NSError* error) const {
   return kAuthenticationErrorCategoryUnknownErrors;
 }