I'm about to introduce the concept of "Loader principal" (as in "the
principal of the CSS loader"), and SheetLoadData already has an
mLoaderPrincipal.

However SheetLoadData's principal is just the triggering principal (the
principal that initiated the load). So name it that with consistency
with SheetInfo::mTriggeringPrincipal etc.

Differential Revision: https://phabricator.services.mozilla.com/D77613