Improve platform support indications for policy documentation.

The generated policy documentation indicates example values and preferences
paths. This change only shows the data relevant for the platforms where the
policy is actually supported.

BUG=None
R=joi@chromium.org, pastarmovj@chromium.org

Review URL: https://codereview.chromium.org/91233003

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@139 7262f16d-afe8-6277-6482-052fa10e57b1
Add support for Chrome policy type that references external data

This CL adds support for a Chrome policy type that references external data.

BUG=256635
TEST=Ran Chrome's target policy_templates
R=joaodasilva@chromium.org

Review URL: https://codereview.chromium.org/92213002

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@140 7262f16d-afe8-6277-6482-052fa10e57b1