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

components/sync/engine_impl/attachments/attachment_downloader_impl.cc

Index: components/sync/engine_impl/attachments/attachment_downloader_impl.cc
diff --git a/components/sync/engine_impl/attachments/attachment_downloader_impl.cc b/components/sync/engine_impl/attachments/attachment_downloader_impl.cc
index 7835c2fe1add8e28e95e3bedcc39f0ec6b81a434..13a69b2c166cb80671e40a680ccbca5d0ea7324a 100644
--- a/components/sync/engine_impl/attachments/attachment_downloader_impl.cc
+++ b/components/sync/engine_impl/attachments/attachment_downloader_impl.cc
@@ -225,7 +225,7 @@ 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);
   }