When deep-cloning a stylesheet (like we do for devtools), we end up with
new (identical) LockedFontFaceRule object, but we'd fail to re-use it,
triggering new font loads. Since we don't cache failed loads, this
caused flashing and a temporary state where the used fonts were not what
this DevTools test expects.
Rejigger the code a little bit to push down the decision of whether to
reuse the FontFaceImpl object further down. That allows us to reuse the
FontFace object for the stylesheet caching / deep cloning use-case.
Differential Revision: https://phabricator.services.mozilla.com/D247704
a0a14bcfb9