Ensure font stylesheets are requested in CORS mode in both AMP and non-AMP documents

[westonruter]

The AMP plugin is currently using the post-processor to add crossorigin=anonymous to font stylesheet links. This is not being done, however, in the non-AMP responses. This is a problem because when a service worker is caching the font stylesheet responses with CORS but then tries to serve it in response to a stylesheet being requested without CORS, then the fetch fails entirely with:

The FetchEvent resulted in a network error response: an "opaque" response was used for a request whose type is not no-cors

So we need to make sure that such font URLs always get served with CORS (crossorigin="anonymous") when in AMP and not-AMP alike to prevent this problem from happening.

Another benefit of CORS is that it ensures that a service worker caching the external stylesheet will not inflate the storage quota. See:

• https://developers.google.com/web/tools/workbox/guides/storage-quota#beware_of_opaque_responses
• https://developers.google.com/web/tools/workbox/guides/handle-third-party-requests#cross-origin_requests_and_opaque_responses

This PR is cherry-picking 02fd0a7 from #1261. We'll not ship the PWA plugin integration for 1.0, but this fix needs to be deployed first.

[amedina]

LGTM.

[kienstra]

This Looks Good

Hi @westonruter,
To echo @amedina, this PR looks good. As expected, a font stylesheet in Twenty Seventeen has crossorigin="anonymous" in both the AMP and non-AMP URLs: