url: trim leading slashes of file URL paths

It should trim the slashes after the colon into three for file URL.

PR-URL: https://github.com/nodejs/node/pull/12507
Refs: https://github.com/w3c/web-platform-tests/pull/5195
Fixes: https://github.com/nodejs/node/issues/11188


Reviewed-By: James M Snell <jasnell@gmail.com>