DevTools: check for importScript statements in compile_frontend.py

This patch adds a check for correct usage of 'importScript' function
in devTools front-end.

BUG=
R=aandrey@chromium.org, vsevik@chromium.org

Review URL: https://codereview.chromium.org/23245006

git-svn-id: svn://svn.chromium.org/blink/trunk@156401 bbb929c8-8fbe-4397-9dbb-9b2b20218538