[Sync] Replacing NULL with nullptr in code and null in comments for components/sync/

components/sync/engine/attachments/attachment_downloader.h

 // 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.
 
 #ifndef COMPONENTS_SYNC_ENGINE_ATTACHMENTS_ATTACHMENT_DOWNLOADER_H_
 #define COMPONENTS_SYNC_ENGINE_ATTACHMENTS_ATTACHMENT_DOWNLOADER_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 22 matching lines @@
   typedef base::Callback<void(const DownloadResult&,
                               std::unique_ptr<Attachment>)>
       DownloadCallback;
 
   virtual ~AttachmentDownloader();
 
   // Download attachment referred by |attachment_id| and invoke |callback| when
   // done.
   //
   // |callback| will receive a DownloadResult code and an Attachment object. If
-  // DownloadResult is not DOWNLOAD_SUCCESS then attachment pointer is NULL.
+  // DownloadResult is not DOWNLOAD_SUCCESS then attachment pointer is null.
   virtual void DownloadAttachment(const AttachmentId& attachment_id,
                                   const DownloadCallback& callback) = 0;
 
   // Create an instance of AttachmentDownloader.
   //
   // |sync_service_url| is the URL of the sync service.
   //
   // |url_request_context_getter| provides a URLRequestContext.
   //
   // |account_id| is the account id to use for downloads.
@@ skipping 13 matching lines @@
       const OAuth2TokenService::ScopeSet scopes,
       const scoped_refptr<OAuth2TokenServiceRequest::TokenServiceProvider>&
           token_service_provider,
       const std::string& store_birthday,
       ModelType model_type);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_ENGINE_ATTACHMENTS_ATTACHMENT_DOWNLOADER_H_