DevTools: fix importScript function to set correct sourceURL

This patch uses window.location fields to generate correct sourceURL
instead of parsing window.location.href manually.

BUG=337151

Review URL: https://codereview.chromium.org/134223004

git-svn-id: svn://svn.chromium.org/blink/trunk@165636 bbb929c8-8fbe-4397-9dbb-9b2b20218538