Consolidate attachment URL construction.

sync/api/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 "sync/api/attachments/attachment_downloader.h"
 
 #include "sync/internal_api/public/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.
 scoped_ptr<AttachmentDownloader> AttachmentDownloader::Create(
-    const std::string& url_prefix,
+    const GURL& sync_service_url,
     const scoped_refptr<net::URLRequestContextGetter>&
         url_request_context_getter,
     const std::string& account_id,
     const OAuth2TokenService::ScopeSet scopes,
     scoped_ptr<OAuth2TokenServiceRequest::TokenServiceProvider>
         token_service_provider) {
   return scoped_ptr<AttachmentDownloader>(
-      new AttachmentDownloaderImpl(url_prefix,
+      new AttachmentDownloaderImpl(sync_service_url,
                                    url_request_context_getter,
                                    account_id,
                                    scopes,
                                    token_service_provider.Pass()));
 }
 
 }  // namespace syncer