Top-level navigation requests use the document fetch destination. CSP's
effective directive algorithm does not list document as a handled fetch
request destination, but our fallback path treated it like an unknown
fetch destination and applied connect-src.
Return no effective fetch directive for document destinations. This lets
top-level navigation use the CSP navigation checks instead. Keep nested
navigation on the existing frame and iframe path, since HTML rewrites
such requests to the container local name when a navigable has a
container.
This makes https://reddit.com/ load instead of redirecting to a blocked
challenge reponse URL.
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