Currently, Fenced Frames disallow all Permission policies for privacy reasons:
https://github.com/WICG/fenced-frame/blob/master/explainer/permission_document_policies.md

This currently breaks the Attribution Reporting API in Fenced Frames which has the permission policy attribution-reporting:
https://wicg.github.io/attribution-reporting-api/#permission-policy-integration

Using Attribution Reporting API in fenced frames is likely essential for some ads use-cases (e.g. using attribution api with FLEDGE). So we should fix this.

Proposal:

We should allow the Attribution Reporting API in "opaque-ads mode" fenced frames by default, and only allow the creating of "opaque-ads mode" Fenced Frames for a particular origin if the Attribution Reporting API is allowed for that origin.

This limits the ability for sites to use "opaque-ads mode" without the Attribution Reporting API, but that seems manageable, given that ads-mode Fenced Frames probably are all interested in the measurement APIs anyway.

See WICG/turtledove#281 for more context.