DevTools: normalize sourceURL paths in importScript function

Currently, importScript function gets scripts to load which contain "../"
in their path. These are written in their sourceURL's, but they should
be actually normalized to match network resource.

R=vsevik, apavlov, pfeldman

Review URL: https://codereview.chromium.org/475073002

git-svn-id: svn://svn.chromium.org/blink/trunk@180355 bbb929c8-8fbe-4397-9dbb-9b2b20218538