This method was previously non-threadsafe due to it needing to access dynamic
URI flags. These flags were used to check the WEbExtensionPolicy to see if the
webextension resource being loaded should be accessible.
Making dynamic URI flags available off-main-thread in general would
unfortunately be quite difficult, due to some of them depending on things like
JS `nsIAboutModule` implementations, so that was not the approach taken.
Instead, all information required is already available in the threadsafe
WebExtensionPolicyCore, which is now directly queried, instead of being queried
indirectly through protocol flags.
Differential Revision: https://phabricator.services.mozilla.com/D215026
5fb9592e42