Can't get css file contents when Drupal base path is not "/"

Problem/Motivation

AMP gets CSS file path from link and import tags, with the URL path and the DRUPAL_ROOT constant. This is enough for an standard Drupal instalation, but when Drupal is served in a second level url (base path is not /), DRUPAL_ROOT is not enough to complete the CSS file path. In such case, Drupal base path should be removed from the URL path.

I attached a patch for a first approximation to fix this problem.

Steps to reproduce

Install a clean Drupal with an standard server path like /var/www/html and configure server to serve the web from a subpath like this http://www.example.com/mysubpath. AMP will try to obtain CSS from /var/www/html/mysubpath.