DevTools: [Sass] Warn users about missing Last-Modified header for css/scss files

When reloading CSS over the network, Last-Modified headers must be served
both for css and scss resources. If not, the CSS updates will not work.

R=vsevik@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/18410003

git-svn-id: svn://svn.chromium.org/blink/trunk@153591 bbb929c8-8fbe-4397-9dbb-9b2b20218538