Bug 1848694 - Remove/avoid global references to nsIIOService r=mccr8,necko-reviewers,kershaw, a=dmeehan

This patch removes the static pointer to nsIIOService in nsContentUtils,
replacing it to calls to mozilla::components::IO::Service.

It also makes nsScriptSecurityManager::sIOService a StaticRefPtr.

Differential Revision: https://phabricator.services.mozilla.com/D188714