[Sync] Removing duplicate includes, part 2.

components/sync/core_impl/attachments/attachment_downloader.cc

 // 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/sync/core/attachments/attachment_downloader.h"
 
-#include <string>
-
 #include "components/sync/core/attachments/attachment_downloader_impl.h"
 
 namespace syncer {
 
 AttachmentDownloader::~AttachmentDownloader() {}
 
 // Factory function for creating AttachmentDownloaderImpl.
 // It is introduced to avoid SYNC_EXPORT-ing AttachmentDownloaderImpl since it
 // inherits from OAuth2TokenService::Consumer which is not exported.
 std::unique_ptr<AttachmentDownloader> AttachmentDownloader::Create(
     const GURL& sync_service_url,
     const scoped_refptr<net::URLRequestContextGetter>&
         url_request_context_getter,
     const std::string& account_id,
     const OAuth2TokenService::ScopeSet scopes,
     const scoped_refptr<OAuth2TokenServiceRequest::TokenServiceProvider>&
         token_service_provider,
     const std::string& store_birthday,
     ModelType model_type) {
   return std::unique_ptr<AttachmentDownloader>(new AttachmentDownloaderImpl(
       sync_service_url, url_request_context_getter, account_id, scopes,
       token_service_provider, store_birthday, model_type));
 }
 
 }  // namespace syncer