Add optional mandatory policy setting for template generation

This CL modifies template writer such that mandatory policy templates are not generated when a flag is set.

CL by kaliamoorthi@chromium.org, reviewed at https://codereview.chromium.org/544113002.

BUG=410856

Review URL: https://codereview.chromium.org/550163002

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@176 7262f16d-afe8-6277-6482-052fa10e57b1