DevTools: Treat sourceURL comment correctly when editing css files.

R=pfeldman, apavlov@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22813003

git-svn-id: svn://svn.chromium.org/blink/trunk@156552 bbb929c8-8fbe-4397-9dbb-9b2b20218538