Breaking out from #1007 which focused specifically on using WP-CLI to do validation of an entire site.

At the moment AMP validation is only performed in response to editing a post, or activating a plugin. There should be passive validation checks performed so that the user can be informed of issues that arise from adding widgets or making other site changes that do not result in validation being performed. WP Cron can be used to initiate revalidation requests for random URLs of enabled templates. This will make the "At a glance" Dashboard widget much more useful on admin as well, as it will actually highlight new issues that the user isn't already immediately aware of:

Relates to #1755 which is for adding a UI in the admin for bulk validation.