Added support for string-enum-list.

Added string-enum-list as a valid policy type. This acts just like 'list' in
terms of the policy templates generated, but the documentation generated is
different (it allows enumerating the valid values).

Landing on behalf of atwilson@chromium.org, reviewed at
https://codereview.chromium.org/352163003.

Review URL: https://codereview.chromium.org/347293003

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@170 7262f16d-afe8-6277-6482-052fa10e57b1