JavaScript module requests (in a non-worker context) always have CORS
enabled. However, CORS requests are only allowed for same-origin or
HTTP/S requests. This patch extends this to allow resource:// requests
from opaque origins (e.g. about: URLs).
We must also set the Access-Control-Allow-Origin header to "null" to
ensure that the response is accepted by the CORS checks. This does not
affect requesting resource:// URLs from resource:// URLs as those are
same-origin and skip CORS checks.
This ultimately enables requesting resource:// JS modules from the
about:settings page.
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