Strip IDS_ as well as IDR_ in the chrome_messages_json generator.

This is needed for renaming string resources in Chrome Remote
Desktop webapp from IDR_ to IDS_ for consistency with other
Chrome string resources.

R=sergeyu@chromium.org

Review URL: https://codereview.chromium.org/138903021

Patch from Lambros Lambrou <lambroslambrou@chromium.org>.

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@148 7262f16d-afe8-6277-6482-052fa10e57b1