Add correctly named members to WebServiceWorkerError::ErrorType.

The original names will be removed in a follow-up patch.

BUG=373594

Review URL: https://codereview.chromium.org/284153002

git-svn-id: svn://svn.chromium.org/blink/trunk@174067 bbb929c8-8fbe-4397-9dbb-9b2b20218538