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

components/sync/engine_impl/attachments/attachment_downloader_impl.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/engine_impl/attachments/attachment_downloader_impl.h"
 
 #include <utility>
 
 #include "base/base64.h"
 #include "base/bind.h"
@@ skipping 207 matching lines @@
   AttachmentUploaderImpl::ConfigureURLFetcherCommon(
       url_fetcher.get(), access_token, raw_store_birthday_, model_type_,
       url_request_context_getter_.get());
   return url_fetcher;
 }
 
 void AttachmentDownloaderImpl::RequestAccessToken(
     DownloadState* download_state) {
   requests_waiting_for_access_token_.push_back(download_state);
   // Start access token request if there is no active one.
-  if (access_token_request_ == NULL) {
+  if (access_token_request_ == nullptr) {
     access_token_request_ = OAuth2TokenServiceRequest::CreateAndStart(
         token_service_provider_.get(), account_id_, oauth2_scopes_, this);
   }
 }
 
 void AttachmentDownloaderImpl::ReportResult(
     const DownloadState& download_state,
     const DownloadResult& result,
     const scoped_refptr<base::RefCountedString>& attachment_data) {
   std::vector<DownloadCallback>::const_iterator iter;
@@ skipping 44 matching lines @@
 
   if (crc32c_raw.size() != sizeof(*crc32c))
     return false;
 
   *crc32c =
       base::NetToHost32(*reinterpret_cast<const uint32_t*>(crc32c_raw.c_str()));
   return true;
 }
 
 }  // namespace syncer