tools: disallow mixed spaces and tabs for indents

Enable eslint rule disallowing mixing tabs and spaces for indentation.
Modify the one file that had been mixing tabs and spaces.

PR-URL: https://github.com/nodejs/node/pull/5135


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>