As for document.fonts, I don't think we intentionally meant to apply
CSP to User/UserAgent fonts. The document certainly has no authority
to block those from loading. (We already have a separate principal
for these which is further evidence that this was unintentional
and we can use the same bit (mUseOriginPrincipal) to avoid CSP.)
Differential Revision: https://phabricator.services.mozilla.com/D111695